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Peavey Valverb

N

N Cook

Jan 1, 1970
0
Valve Reverb, 1U case prob. 1994 , no output.
One obvious problem. Seems to be C-R-C-R-C HT ps with
the first R overheated to partially white body and O/C.
Scraped back and remnants measure 4K and 12 K. Comparing
with other MO 1/2 W Rs, band positions on bodies, the band heated to black
was
probably gold and the first 2 Brown then blue , third is gone totally, .
so maybe 16K .
Supplying HT to 3 valves 12AT7 and 12AX7 so common enough.
Anyone know the correct value of this R, The second one in
the ps drop down chain is 22K and looks fine?

I've not dug out my Avo valve tester yet, as not really a valve-man, so any
ideas for this fault?
The wax around the 3 HV Cs is unmelted but as the 22K is fine at the moment
I suspect a powered-up-only short in middle 47uF,500V electro.

Apparently no separate secondary for heater voltage - seems to use the
same LV winding used for the transitor power rails - is that so ?

electronic hints and repair briefs , schematics/manuals list on
http://homepages.tcp.co.uk/~diverse
 
A

Asimov

Jan 1, 1970
0
"N Cook" bravely wrote to "All" (01 Dec 04 18:03:31)
--- on the heady topic of "Peavey Valverb"

The R's are often the same in the ps divider chain. Burnt pigment
tends to toast to a different shade but if you found some original
paint then 16K is still in the ballpark. Replace with flameproof type
for safety. The resistors are used mainly for RC hum filtering with
low DC current present; probably not more than about 2 or 3 mA total.

The dual tube heaters can use 12 volts in series or 6 volts in
parallel so it doesn't matter. The HT caps are encased in wax? Hmmm,
pretty odd for a 1994 circuit. Certainly check the caps for leakage
but beware of the voltage, it really bites! They might simply need a
slow reforming as electros are self-healing, given half a chance.

A*s*i*m*o*v


NC> From: "N Cook" <[email protected]>

NC> Valve Reverb, 1U case prob. 1994 , no output.
NC> One obvious problem. Seems to be C-R-C-R-C HT ps with
NC> the first R overheated to partially white body and O/C.
NC> Scraped back and remnants measure 4K and 12 K. Comparing
NC> with other MO 1/2 W Rs, band positions on bodies, the band heated to
NC> black was
NC> probably gold and the first 2 Brown then blue , third is gone totally,
NC> . so maybe 16K .
NC> Supplying HT to 3 valves 12AT7 and 12AX7 so common enough.
NC> Anyone know the correct value of this R, The second one in
NC> the ps drop down chain is 22K and looks fine?

NC> I've not dug out my Avo valve tester yet, as not really a valve-man,
NC> so any ideas for this fault?
NC> The wax around the 3 HV Cs is unmelted but as the 22K is fine at the
NC> moment I suspect a powered-up-only short in middle 47uF,500V electro.

NC> Apparently no separate secondary for heater voltage - seems to use the
NC> same LV winding used for the transitor power rails - is that so ?

.... New computer? But I like my vacuum tubes... They keep me warm.
 
N

N Cook

Jan 1, 1970
0
Asimov said:
"N Cook" bravely wrote to "All" (01 Dec 04 18:03:31)
--- on the heady topic of "Peavey Valverb"

The R's are often the same in the ps divider chain. Burnt pigment
tends to toast to a different shade but if you found some original
paint then 16K is still in the ballpark. Replace with flameproof type
for safety. The resistors are used mainly for RC hum filtering with
low DC current present; probably not more than about 2 or 3 mA total.

The dual tube heaters can use 12 volts in series or 6 volts in
parallel so it doesn't matter. The HT caps are encased in wax? Hmmm,
pretty odd for a 1994 circuit. Certainly check the caps for leakage
but beware of the voltage, it really bites! They might simply need a
slow reforming as electros are self-healing, given half a chance.

A*s*i*m*o*v


NC> From: "N Cook" <[email protected]>

NC> Valve Reverb, 1U case prob. 1994 , no output.
NC> One obvious problem. Seems to be C-R-C-R-C HT ps with
NC> the first R overheated to partially white body and O/C.
NC> Scraped back and remnants measure 4K and 12 K. Comparing
NC> with other MO 1/2 W Rs, band positions on bodies, the band heated to
NC> black was
NC> probably gold and the first 2 Brown then blue , third is gone totally,
NC> . so maybe 16K .
NC> Supplying HT to 3 valves 12AT7 and 12AX7 so common enough.
NC> Anyone know the correct value of this R, The second one in
NC> the ps drop down chain is 22K and looks fine?

NC> I've not dug out my Avo valve tester yet, as not really a valve-man,
NC> so any ideas for this fault?
NC> The wax around the 3 HV Cs is unmelted but as the 22K is fine at the
NC> moment I suspect a powered-up-only short in middle 47uF,500V electro.

NC> Apparently no separate secondary for heater voltage - seems to use the
NC> same LV winding used for the transitor power rails - is that so ?

... New computer? But I like my vacuum tubes... They keep me warm.

I'm wondering because the R is discoloured like long term over-heating ie
gone white if that middle C is ok and just replace with flameproof higher
wattage 16K
 
N

N Cook

Jan 1, 1970
0
Asimov said:
"N Cook" bravely wrote to "All" (01 Dec 04 18:03:31)
--- on the heady topic of "Peavey Valverb"

The R's are often the same in the ps divider chain. Burnt pigment
tends to toast to a different shade but if you found some original
paint then 16K is still in the ballpark. Replace with flameproof type
for safety. The resistors are used mainly for RC hum filtering with
low DC current present; probably not more than about 2 or 3 mA total.

The dual tube heaters can use 12 volts in series or 6 volts in
parallel so it doesn't matter. The HT caps are encased in wax? Hmmm,
pretty odd for a 1994 circuit. Certainly check the caps for leakage
but beware of the voltage, it really bites! They might simply need a
slow reforming as electros are self-healing, given half a chance.

A*s*i*m*o*v


NC> From: "N Cook" <[email protected]>

NC> Valve Reverb, 1U case prob. 1994 , no output.
NC> One obvious problem. Seems to be C-R-C-R-C HT ps with
NC> the first R overheated to partially white body and O/C.
NC> Scraped back and remnants measure 4K and 12 K. Comparing
NC> with other MO 1/2 W Rs, band positions on bodies, the band heated to
NC> black was
NC> probably gold and the first 2 Brown then blue , third is gone totally,
NC> . so maybe 16K .
NC> Supplying HT to 3 valves 12AT7 and 12AX7 so common enough.
NC> Anyone know the correct value of this R, The second one in
NC> the ps drop down chain is 22K and looks fine?

NC> I've not dug out my Avo valve tester yet, as not really a valve-man,
NC> so any ideas for this fault?
NC> The wax around the 3 HV Cs is unmelted but as the 22K is fine at the
NC> moment I suspect a powered-up-only short in middle 47uF,500V electro.

NC> Apparently no separate secondary for heater voltage - seems to use the
NC> same LV winding used for the transitor power rails - is that so ?

... New computer? But I like my vacuum tubes... They keep me warm.

I've now dismantled.
Wax coloured hot-melt glue, not wax.
What I thought was just an intermediary smoothing stage is actually also a
feed to a 12AT7 via a presumably impedance matching output transformer
to A2 anode and the other via 47R to the A1.
This transformer unpowered resistance seems ok about 2K on high imp
side and 300 on low imp which directly connects to one of the coil
type transducers on the spring-line. So maybe excess current in this valve
hopefully rather than a failing impedance matching transformer.

I've only ever come across piezo transducers on spring-lines before
this is 56R on the send? transformer connected side and 196R on receive end
coils.
 
A

Asimov

Jan 1, 1970
0
"N Cook" bravely wrote to "All" (02 Dec 04 07:40:55)
--- on the heady topic of "Re: Peavey Valverb"

NC> From: "N Cook said:
The R's are often the same in the ps divider chain. Burnt pigment
tends to toast to a different shade but if you found some original
paint then 16K is still in the ballpark. Replace with flameproof type
for safety. The resistors are used mainly for RC hum filtering with
low DC current present; probably not more than about 2 or 3 mA total.


NC> I'm wondering because the R is discoloured like long term over-heating
NC> ie gone white if that middle C is ok and just replace with flameproof
NC> higher wattage 16K

I would beware of replacing with higher wattage since ps dropping
resistors often have a dual purpose as fuses. If something goes wrong,
using a higher wattage resistor may result in worse damage or even a
fire hazard.

A*s*i*m*o*v

.... Thomas Edison invented the "Light Emitting Resistor"
 
A

Asimov

Jan 1, 1970
0
"N Cook" bravely wrote to "All" (02 Dec 04 15:36:59)
--- on the heady topic of "Re: Peavey Valverb"

NC> I've now dismantled.
NC> Wax coloured hot-melt glue, not wax.
NC> What I thought was just an intermediary smoothing stage is actually
NC> also a feed to a 12AT7 via a presumably impedance matching output
NC> transformer to A2 anode and the other via 47R to the A1.
NC> This transformer unpowered resistance seems ok about 2K on high imp
NC> side and 300 on low imp which directly connects to one of the coil
NC> type transducers on the spring-line. So maybe excess current in this
NC> valve hopefully rather than a failing impedance matching transformer.

NC> I've only ever come across piezo transducers on spring-lines before
NC> this is 56R on the send? transformer connected side and 196R on
NC> receive end coils.

Check the input coupling cap to the grid of the 12AT7 for leakage.
If this cap leaks it would cause a large current through the tube.
If the transformer shorted to ground, perhaps you can isolate it?

A*s*i*m*o*v

.... 'Keep the smoke inside.' -- 1st Rule of Electronics.
 
N

N Cook

Jan 1, 1970
0
"N Cook" bravely wrote to "All" (02 Dec 04 07:40:55)
--- on the heady topic of "Re: Peavey Valverb"

NC> From: "N Cook said:
The R's are often the same in the ps divider chain. Burnt pigment
tends to toast to a different shade but if you found some original
paint then 16K is still in the ballpark. Replace with flameproof type
for safety. The resistors are used mainly for RC hum filtering with
low DC current present; probably not more than about 2 or 3 mA total.


NC> I'm wondering because the R is discoloured like long term over-heating
NC> ie gone white if that middle C is ok and just replace with flameproof
NC> higher wattage 16K

I would beware of replacing with higher wattage since ps dropping
resistors often have a dual purpose as fuses. If something goes wrong,
using a higher wattage resistor may result in worse damage or even a
fire hazard.

A*s*i*m*o*v

.... Thomas Edison invented the "Light Emitting Resistor"

I replaced with same wattage R ,disconnected valves and cut trace to
matching transformer.
Powered up via variac to full, giving unloaded HT rails of 440V or so
sustained with no problems.
On load hopefully the 440V should drop to less than design
max of 300V.

Dug out my old Avo 160 valve tester for the first time in 10 years, noticed
last
calibrated in 1984 for the REME in the Hebrides. Also powered up via
variac initially. All those thumbwheel switch contacts and other contacts
that
could easily corrode but no problems.
I'd forgotten about the sequence , balance mains V, heater check, cold
insulation then hot
insulation , Test and Gasified check. Also forgotten that nifty 'telephone
dialler' for set mA/V.
All 3 valves nicely in the green GOOD sector. For the 12AT7 design
value is 5.5 mA/V and on test is 5.4 and 5.9 for the other triode.

Ignoring the drop associated with the other 2 valves and not
at the moment knowing the actual anode current, if at the test
value of 10mA then drop over 16K would give a dissipation of
1.6W. I cannot find in the data book or tester manual whether
that is 10mA each triode or combined value but I assume it
is per triode and 20mA if both sections on at test levels so 3.2W.

So leaves either problem with output matching transformer or long term heat
damage
to that dropper, under-rated ?. Actually 1W rating not 0.5W as I originally
said
Although originally light blue-grey body was white suggesting
long term over-heating I would have expected the resistance bits to
have migrated from 4K + 12K = 16K.

Where to next?

electronic hints and repair briefs , schematics/manuals list on
http://homepages.tcp.co.uk/~diverse
 
A

Asimov

Jan 1, 1970
0
"N Cook" bravely wrote to "All" (03 Dec 04 17:55:30)
--- on the heady topic of "Re: Peavey Valverb"

NC> From: "N Cook" <[email protected]>

NC> I replaced with same wattage R ,disconnected valves and cut trace to
NC> matching transformer.
NC> Powered up via variac to full, giving unloaded HT rails of 440V or so
NC> sustained with no problems.
NC> On load hopefully the 440V should drop to less than design
NC> max of 300V.

NC> Dug out my old Avo 160 valve tester for the first time in 10 years,
NC> noticed last
NC> calibrated in 1984 for the REME in the Hebrides. Also powered up via
NC> variac initially. All those thumbwheel switch contacts and other
NC> contacts that
NC> could easily corrode but no problems.
NC> I'd forgotten about the sequence , balance mains V, heater check, cold
NC> insulation then hot
NC> insulation , Test and Gasified check. Also forgotten that nifty
NC> 'telephone dialler' for set mA/V.
NC> All 3 valves nicely in the green GOOD sector. For the 12AT7 design
NC> value is 5.5 mA/V and on test is 5.4 and 5.9 for the other triode.

NC> Ignoring the drop associated with the other 2 valves and not
NC> at the moment knowing the actual anode current, if at the test
NC> value of 10mA then drop over 16K would give a dissipation of
NC> 1.6W. I cannot find in the data book or tester manual whether
NC> that is 10mA each triode or combined value but I assume it
NC> is per triode and 20mA if both sections on at test levels so 3.2W.

NC> So leaves either problem with output matching transformer or long term
NC> heat damage
NC> to that dropper, under-rated ?. Actually 1W rating not 0.5W as I
NC> originally said

Aha! Well the 440 volts will also drop somewhat so that the plate will
be within that 300 volts spec. So now 16K ohms at 1 watt would be
about 8mA. That makes sense.


NC> Although originally light blue-grey body was white suggesting
NC> long term over-heating I would have expected the resistance bits to
NC> have migrated from 4K + 12K = 16K.

NC> Where to next?

Check the 12AT7's cathode bias resistor for a bypass electro about
47uF or 100uF if it isn't shorted or very leaky.

If your PS caps aren't shorted, the tubes are good, and the coupling
caps aren't leaky, then it should be working just fine. Use the variac
and monitor the plate voltage. Don't let the smoke out... ;-)

A*s*i*m*o*v

.... 'Keep the smoke inside.' -- 1st Rule of Electronics.
 
N

N Cook

Jan 1, 1970
0
Asimov said:
"N Cook" bravely wrote to "All" (02 Dec 04 15:36:59)
--- on the heady topic of "Re: Peavey Valverb"

NC> I've now dismantled.
NC> Wax coloured hot-melt glue, not wax.
NC> What I thought was just an intermediary smoothing stage is actually
NC> also a feed to a 12AT7 via a presumably impedance matching output
NC> transformer to A2 anode and the other via 47R to the A1.
NC> This transformer unpowered resistance seems ok about 2K on high imp
NC> side and 300 on low imp which directly connects to one of the coil
NC> type transducers on the spring-line. So maybe excess current in this
NC> valve hopefully rather than a failing impedance matching transformer.

NC> I've only ever come across piezo transducers on spring-lines before
NC> this is 56R on the send? transformer connected side and 196R on
NC> receive end coils.

Check the input coupling cap to the grid of the 12AT7 for leakage.
If this cap leaks it would cause a large current through the tube.
If the transformer shorted to ground, perhaps you can isolate it?

A*s*i*m*o*v

... 'Keep the smoke inside.' -- 1st Rule of Electronics.

The grid seems to be isolated via Vactec LED-photocell coupler/isolator.

Reconnected 12AT7 HT line and poweed up on 40 percent mains
and 200 to 20 V drop across the 16K for a few seconds,
same without the valve in place.

I suppose I never come across primary to secondary shorts on transformers
and never thought to put a DVM across this output matching T.
2.14K resistance on HV coil and 310R on output to springline
coil which is ground referenced. Anyway min. 150R between these 2 coil
terminals
and
I assume not some sort of auto transformer. Desoldered this T,
for the archives the frame is not grounded but the tangs are bent over and
soldered against the pcb polyester but this is not obvious.
About 1 inch cube of small transformer. The 'bobbin' or at least axial
end faces are thin plastic and could easily be bent back showing
large coil to centre and smaller on outside with vinyl tape between
the two and loose coils of wire at the ends. Not constrained really ,
certainly not lacquered or varnished just some liquid yellow paint or
something, maybe decomposed yellow vinyl tape..
Anyway probing about and pushing the coil bulk, this cross-connection
resistance
would increase above 30M and putting a cable tie around the coil bulk
would at least temporarily 'cure' the problem.
Assuming the iron laminations will part it will probably be
fairly straightforward to count off and rewind the outer coil or
at least properly seat the existing coils and lacquer in place.

Badly designed / made output matching transformer looks like.

electronic hints and repair briefs , schematics/manuals list on
http://homepages.tcp.co.uk/~diverse
 
A

Asimov

Jan 1, 1970
0
"N Cook" bravely wrote to "All" (06 Dec 04 12:00:12)
--- on the heady topic of "Re: Peavey Valverb"

NC> From: "N Cook" <[email protected]>
NC> Subject: Re: Peavey Valverb

NC> I suppose I never come across primary to secondary shorts on
NC> transformers and never thought to put a DVM across this output matching
NC> T. 2.14K resistance on HV coil and 310R on output to springline
NC> coil which is ground referenced. Anyway min. 150R between these 2 coil
NC> terminals
NC> and
NC> I assume not some sort of auto transformer. Desoldered this T,
NC> for the archives the frame is not grounded but the tangs are bent over
NC> and soldered against the pcb polyester but this is not obvious.
NC> About 1 inch cube of small transformer. The 'bobbin' or at least axial
NC> end faces are thin plastic and could easily be bent back showing
NC> large coil to centre and smaller on outside with vinyl tape between
NC> the two and loose coils of wire at the ends. Not constrained really ,
NC> certainly not lacquered or varnished just some liquid yellow paint or
NC> something, maybe decomposed yellow vinyl tape..
NC> Anyway probing about and pushing the coil bulk, this cross-connection
NC> resistance
NC> would increase above 30M and putting a cable tie around the coil bulk
NC> would at least temporarily 'cure' the problem.
NC> Assuming the iron laminations will part it will probably be
NC> fairly straightforward to count off and rewind the outer coil or
NC> at least properly seat the existing coils and lacquer in place.

NC> Badly designed / made output matching transformer looks like.

Looks like the transformer wasn't properly rated for voltage or it was
slightly roasted by an over-current condition. It could be a pretty
common part. From my experience these little matching transformers
tend to have little bass response below about 200Hz which is a good
thing for reverb spring lines. This means just about any type in that
ballpark range would work well. The winding resistances of 2.14K and
310R is a ratio of about 6.9:1 and is probably close to the turns
ratio because the wire, for such low power applications, is sometimes
the same gauge in both windings. "But" it is only a guess.

If you will use a tie-wrap, at least add an in-line low current fuse
to the ps tap, as one can't be sure the xfrmr won't short again.

A*s*i*m*o*v

.... Dunno if we'll get that past the CSA und UL 'owever.
 
N

N Cook

Jan 1, 1970
0
E & I laminations came apart easily as waxed in not varnish

Wire is about 3 thou diameter so assuming 44SWG so 0.093 lb and 2,985
ohms per 1000 yards and averaged coil
circumferences then the outer bunch is about 1250 turns by
resistance or 1550 turns by weight so about 9,000 turns on the HV section.
So i will have to dig out and maintain my old Eta Tool company , Leicester,
coil winder as the reversing clutch is jumping and the differential gearing
jokey tyre is perished. Must find out how old this lump is.
Douglas coil winders always looked more elegant.
 
N

N Cook

Jan 1, 1970
0
Asimov said:
"N Cook" bravely wrote to "All" (06 Dec 04 12:00:12)
--- on the heady topic of "Re: Peavey Valverb"

NC> From: "N Cook" <[email protected]>
NC> Subject: Re: Peavey Valverb

NC> I suppose I never come across primary to secondary shorts on
NC> transformers and never thought to put a DVM across this output matching
NC> T. 2.14K resistance on HV coil and 310R on output to springline
NC> coil which is ground referenced. Anyway min. 150R between these 2 coil
NC> terminals
NC> and
NC> I assume not some sort of auto transformer. Desoldered this T,
NC> for the archives the frame is not grounded but the tangs are bent over
NC> and soldered against the pcb polyester but this is not obvious.
NC> About 1 inch cube of small transformer. The 'bobbin' or at least axial
NC> end faces are thin plastic and could easily be bent back showing
NC> large coil to centre and smaller on outside with vinyl tape between
NC> the two and loose coils of wire at the ends. Not constrained really ,
NC> certainly not lacquered or varnished just some liquid yellow paint or
NC> something, maybe decomposed yellow vinyl tape..
NC> Anyway probing about and pushing the coil bulk, this cross-connection
NC> resistance
NC> would increase above 30M and putting a cable tie around the coil bulk
NC> would at least temporarily 'cure' the problem.
NC> Assuming the iron laminations will part it will probably be
NC> fairly straightforward to count off and rewind the outer coil or
NC> at least properly seat the existing coils and lacquer in place.

NC> Badly designed / made output matching transformer looks like.

Looks like the transformer wasn't properly rated for voltage or it was
slightly roasted by an over-current condition. It could be a pretty
common part. From my experience these little matching transformers
tend to have little bass response below about 200Hz which is a good
thing for reverb spring lines. This means just about any type in that
ballpark range would work well. The winding resistances of 2.14K and
310R is a ratio of about 6.9:1 and is probably close to the turns
ratio because the wire, for such low power applications, is sometimes
the same gauge in both windings. "But" it is only a guess.

If you will use a tie-wrap, at least add an in-line low current fuse
to the ps tap, as one can't be sure the xfrmr won't short again.

A*s*i*m*o*v

... Dunno if we'll get that past the CSA und UL 'owever.

For the archives: Repaired and write up with other Valverb info will be on
URL below by end of 2004

electronic hints and repair briefs , schematics/manuals list on
http://homepages.tcp.co.uk/~diverse
 
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