Vox JMI AC10 SRT in the house! (low plate voltage)

[Yeatzee]

This thread will be much more fun and interesting in the near future I promise! But for now I want check with you guys to see what you think about this amplifier's low plate voltage. I purchased it from a dealer who said it was recently serviced and biased so I was hoping it'd be plug and play, but alas that's never the case for me. Because it was purchased from a dealer, I'm in a holding pattern waiting to hear what they want to do before we dive deeper so this is all theoretical for now.

The amp is a real deal JMI AC10 SRT, of the Domino variety which if you don't know is just an SRT chassis installed in a different cabinet. The amp has had a great deal of work done to it, but the original WIMA caps are there as are the transformers. Resistors have just about all been replaced with CF, and the electrolytics have also been replaced (mostly).

I got a schematic of the amp (the right one as I understand it), and dived in checking everything to make sure it's all healthy and the work done to it wasn't too goofy. There are few things noted on there we'll circle back to, but for now I want to focus on the issue that's making me pause.





The amp is biased cool, and has low plate voltage already compared to the schematic. I'm worried it wont' be able to bias correctly without extremely low plate voltages as-is, so my hunch is there's something else that's wrong.

Here's what I'm getting with 119vac from the wall:

Cathode Resistor: 133.4 (stock value)
Voltage across resistor: 9.85vdc
Plate to cathode voltage: 264vdc

This puts us at around 75% dissipation. I would expect with a stock value resistor and modern wall voltages to be over 100%, not way under. Schematic shows plate voltage as 290vdc, but again that's expecting lower wall voltage right?

My concern is if the resistor value is dropped, won't the plate voltage drop significantly as well? We could be in the low 200's to get it around 100%, so something has to be off.

I tried a different EZ81 from another one of my amps, no real change. I tried different EL84's, no real change. The bias cap is also a replacement. The can cap connected to the rectifier cathode is a dual 50uF replacement weber (32uF original value). The rest of the main electrolytics consist of an original triple 16uF can cap with added 33uF caps wired in parallel of each node for I guess additional capacitance. Not really sure.

Heaters measure 6.4v so they're fine. Input side of the EZ81 measured 314vac vs 300vac in the schematic so PT likely fine?

Likely causes I can think of are failed/failing filter caps (even though it seems like a modern replacement in the first node?) and/or 2 bad EZ81's. Anything else I'm missing?

[mresistor]

Did you already say this? What are the plate voltages with no power tubes installed?

[Yeatzee]

Did you already say this? What are the plate voltages with no power tubes installed?

390vdc range

[mresistor]

Have you tried another EL84 duet?

[Yeatzee]

Have you tried another EL84 duet?

Yeah I tried several sets, same basic results.

[HotBluePlates]

... The amp is a real deal JMI AC10 SRT, of the Domino variety ...

Here's what I'm getting with 119vac from the wall:

Cathode Resistor: 133.4 (stock value)
Voltage across resistor: 9.85vdc
Plate to cathode voltage: 264vdc

This puts us at around 75% dissipation. I would expect with a stock value resistor and modern wall voltages to be over 100%, not way under. Schematic shows plate voltage as 290vdc, but again that's expecting lower wall voltage right? ...

I also have a Domino Super Reverb.  Give me a little time to measure mine for comparison.

[AlNewman]

So are all those pre amp filter caps just the original paper in oil caps in parallel with new 33uf?  Your drawing sort of points that way.  Would they have added new caps in parallel because they were leaking DC? 
Is the 319 VAC the measure you get without the rectifier in?

[AlNewman]

You can measure the voltage drop across the 2 10k and the 22k dropping resistors on your little tubes, and maybe 1 or more is out to lunch.  They are 12au7, so they can see higher current than normal, but it doesn't look like they're designed that way.  If you get much more than a few mA through those resistors, then I would suspect the original caps.

[Yeatzee]

So are all those pre amp filter caps just the original paper in oil caps in parallel with new 33uf?  Your drawing sort of points that way.  Would they have added new caps in parallel because they were leaking DC? 
Is the 319 VAC the measure you get without the rectifier in?

The wima's? They're TFF not PIO. The 33uF 350v caps that were added are in parallel with the triple 16uF can cap in the schematic. The Wima's aren't leaking I don't think.

[AlNewman]

So the cap can is original then?
Same difference, I'd measure the current at your dropping resistors in comparison with the cathode resistors at each node and see if the numbers add up.

[astronomicum]

The schematic voltages are not making sense to me. A 390V supply with 295V on the plates with lets say a 200 ohm DC resistance across half the 8K OT would be 475mA. A 390V supply with 300V on the screens across a 1K is 90mA. What am I missing?

[HotBluePlates]

Cathode Resistor: 133.4 (stock value)

This resistor itself looks like the original part form when the amp was made.  So regardless of the schematic, I'd tend to call "135Ω" the right value.

FWIW, my own amp was worked on before I got it, and the previous tech paralleled 2 resistors to trim it to about 130Ω.

The schematic voltages are not making sense to me. A 390V supply with 295V on the plates ... A 390V supply with 300V on the screens across a 1K is 90mA. What am I missing?

The schematic shows the output transformer center-tap taking voltage from the EZ81 output & 1st filter cap.

However look at the chassis photo, and the center-tap (Red wire) is connected to Pin 9 (G2) of one of the EL84s.  Follow the Yellow wire also attached to that pin, and you'll see this is after the 1kΩ resistor between the 1st and 2nd filter caps.

Turns out the output transformer & power tube plates come after the 1kΩ resistor, hence the big voltage drop.


I checked my amp, and the OT center-tap is after the 1kΩ resistor.  I measured voltages as follows:

Outlet Voltage: 117v AC

Rectifier Output:  371v DC
2nd Filter Cap (after 1kΩ):  286v DC

EL84 Screen (Pin 9):  286v DC
EL84 Plate (Pin 7):  281v DC

EL84 Cathode (Pin 3):  10.43v DC
Cathode Resistor:  130.2Ω
Cathode Current:  10.43v DC / 130.2Ω = 80.11mA

Idle ~10.8 watts per tube (not correcting for screen current)

Drop Across 1kΩ:  (371v - 286v) / 1kΩ = ~85mA ---> 5mA for the preamp, 80mA for EL84 plates & screens.

The amp is biased cool, and has low plate voltage already compared to the schematic. I'm worried it wont' be able to bias correctly without extremely low plate voltages as-is, so my hunch is there's something else that's wrong.

Looks like the schematic is wrong.  About 290vdc seems right (and this amp gets plenty-loud with that voltage, which is not-far from the max EL84s were supposed to get according to their data sheets).

As for low plate current, I'd say make sure you weren't plugged into a lightbulb limiter while taking readings.  I initially had my amp connected to one, and was getting about 7.2v across the cathode resistor, and more like 220v DC at the EL84 plates.

If you didn't have a lightbulb limiter in place, I'd suggest trying a different pair of EL84s.  You may find others bring the plate current & idle dissipation up.

And remember, if the amp sounds good, then it is good.  You can tinker out of curiosity, but if it were my amp I would probably leave it as-is.

[Yeatzee]

Cathode Resistor: 133.4 (stock value)

This resistor itself looks like the original part form when the amp was made.  So regardless of the schematic, I'd tend to call "135Ω" the right value.

FWIW, my own amp was worked on before I got it, and the previous tech paralleled 2 resistors to trim it to about 130Ω.

The schematic voltages are not making sense to me. A 390V supply with 295V on the plates ... A 390V supply with 300V on the screens across a 1K is 90mA. What am I missing?

The schematic shows the output transformer center-tap taking voltage from the EZ81 output & 1st filter cap.

However look at the chassis photo, and the center-tap (Red wire) is connected to Pin 9 (G2) of one of the EL84s.  Follow the Yellow wire also attached to that pin, and you'll see this is after the 1kΩ resistor between the 1st and 2nd filter caps.

Turns out the output transformer & power tube plates come after the 1kΩ resistor, hence the big voltage drop.


I checked my amp, and the OT center-tap is after the 1kΩ resistor.  I measured voltages as follows:

Outlet Voltage: 117v AC

Rectifier Output:  371v DC
2nd Filter Cap (after 1kΩ):  286v DC

EL84 Screen (Pin 9):  286v DC
EL84 Plate (Pin 7):  281v DC

EL84 Cathode (Pin 3):  10.43v DC
Cathode Resistor:  130.2Ω
Cathode Current:  10.43v DC / 130.2Ω = 80.11mA

Idle ~10.8 watts per tube (not correcting for screen current)

Drop Across 1kΩ:  (371v - 286v) / 1kΩ = ~85mA ---> 5mA for the preamp, 80mA for EL84 plates & screens.

The amp is biased cool, and has low plate voltage already compared to the schematic. I'm worried it wont' be able to bias correctly without extremely low plate voltages as-is, so my hunch is there's something else that's wrong.

Looks like the schematic is wrong.  About 290vdc seems right (and this amp gets plenty-loud with that voltage, which is not-far from the max EL84s were supposed to get according to their data sheets).

As for low plate current, I'd say make sure you weren't plugged into a lightbulb limiter while taking readings.  I initially had my amp connected to one, and was getting about 7.2v across the cathode resistor, and more like 220v DC at the EL84 plates.

If you didn't have a lightbulb limiter in place, I'd suggest trying a different pair of EL84s.  You may find others bring the plate current & idle dissipation up.

And remember, if the amp sounds good, then it is good.  You can tinker out of curiosity, but if it were my amp I would probably leave it as-is.

I appreciate you spelling it all out for me, extremely helpful as always. I will circle back tonight and retake voltages to confirm the parity between current readings.

I briefly played with dropping the cathode resistor and at ~75ohm I'm getting a plate to cathode voltage of around 235vdc and about 7 or so volts across the resistor. This puts me right around 90% or so, so closer to what I assumed was always the target with vox amps like this but I guess it's not as cut and dry as I had thought...

The schematic is what's really throwing me off, having a good 50+ volts less on the plates with a bias around 90% seems like it'd be wrong. What I can say is I played around with the amp today, and got lost in it for 2 hours. It's an incredible sounding thing, really really reminds me of my TB AC30 Super Twin with similar over the top levels of gain when I played with the preamp tubes. It's got a lot of weight to the tone as well, despite being lower wattage (though it's a loud 10w for sure), when running through the Super Twin's dual alnico silver 2x12. I'm in a waiting pattern to make changes to the amp, but once I get the green light it'll be great to dial it in some more and hopefully make it fully healthy.

[HotBluePlates]

The schematic is what's really throwing me off, having a good 50+ volts less on the plates ...

The more I measure stock vintage amps, the more I come to realize schematics are only a "guide" and not absolute-truth.

   - I believe it's a mistake to assume the schematics have precise voltage values, at least in musical instrument amps.

   - I think I see others assume that being "on spec" equates to a degree of goodness, and so they attempt to exactly match a schematic.  Perhaps we do this because we don't know why a particular amp sounds good, so this seems like an objective target we can match to "increase goodness."

Have you tried another EL84 duet?

Yeah I tried several sets, same basic results.

Here's what I'm getting with 119vac from the wall:

Cathode Resistor: 133.4 (stock value)
Voltage across resistor: 9.85vdc
Plate to cathode voltage: 264vdc

If the cathode bypass cap of the EL84s was a partial-short-circuit, it would cause the EL84 cathode voltage to measure low even as the EL84 cathode current is very high.  The high current would also drag down the B+ volts at the EL84 plates.

What about trying to have only the cathode resistor present, instead of being bypassed?  I understand that as it sits, this will be difficult because the last guy chose to use the cap's leads as the mounting posts for the resistor.

[Yeatzee]

The schematic is what's really throwing me off, having a good 50+ volts less on the plates ...

The more I measure stock vintage amps, the more I come to realize schematics are only a "guide" and not absolute-truth.

   - I believe it's a mistake to assume the schematics have precise voltage values, at least in musical instrument amps.

   - I think I see others assume that being "on spec" equates to a degree of goodness, and so they attempt to exactly match a schematic.  Perhaps we do this because we don't know why a particular amp sounds good, so this seems like an objective target we can match to "increase goodness."

Have you tried another EL84 duet?

Yeah I tried several sets, same basic results.

Here's what I'm getting with 119vac from the wall:

Cathode Resistor: 133.4 (stock value)
Voltage across resistor: 9.85vdc
Plate to cathode voltage: 264vdc

IF the cathode bypass cap of the EL84s was a partial-short-circuit, it would cause the EL84 cathode voltage to measure low even as the EL84 cathode current is very high.  The high current would also drag down the B+ volts at the EL84 plates.

What about trying to have only the cathode resistor present, instead of being bypassed?  I understand that as it sits, this will be difficult because the last guy chose to use the cap's leads as the mounting posts for the resistor.

Yeah I decided to test that too with a new cap/resistor temp'd in. I redid all of the measurements following your formatting with a new set of tungsol EL84's and here's what I got:

Outlet Voltage: 117v AC 119.6vAC

Rectifier Output:  371v DC 362vDC
2nd Filter Cap (after 1kΩ):  286v DC 248vDC

EL84 Screen (Pin 9):  286v DC 248vDC
EL84 Plate (Pin 7):  281v DC 239vDC

EL84 Cathode (Pin 3):  10.43v DC 7.25vDC
Cathode Resistor:  130.2Ω 74.8Ω
Cathode Current:  10.43v DC / 130.2Ω = 80.11mA 96.9mA

Drop Across 1kΩ:  (371v - 286v) / 1kΩ = ~85mA ---> 5mA for the preamp, 80mA for EL84 plates & screens.
(362v - 248v) / 1000Ω = 114mA

[HotBluePlates]

Cathode Resistor:  130.2Ω 74.8Ω

IMO, this is an enormous mistake, since the "right value" for a pair of EL84 is closer to 130Ω.  I recommend you use the 135Ω resistor in the amp for all further testing.

... I redid all of the measurements following your formatting with a new set of tungsol EL84's and here's what I got:

...
Cathode Current:  10.43v DC / 130.2Ω = 80.11mA 96.9mA

Drop Across 1kΩ:  (371v - 286v) / 1kΩ = ~85mA ---> 5mA for the preamp, 80mA for EL84 plates & screens.
(362v - 248v) / 1000Ω = 114mA

114mA overall minus 96.9mA for the output tubes ---> 17.1mA in the preamp.

There's no way your preamp should be pulling that much current.  AlNewman seems to have a point that the original can capacitor may be leaking current & pulling the supply low.

FWIW, my amp was worked on prior to my getting it.  The internal 3-section can was removed, and a terminal strip installed in its place.  I do have filter caps sprinkled throughout the preamp like your amp, but I don't have the old cap-sections being used as tie-points.  Perhaps evicting that old can is a first order of business.

[shooter]

2nd Filter Cap (after 1kΩ):  286v DC 248vDC

these seem to indicate the drag on the PS

[Yeatzee]

Cathode Resistor:  130.2Ω 74.8Ω

IMO, this is an enormous mistake, since the "right value" for a pair of EL84 is closer to 130Ω.  I recommend you use the 135Ω resistor in the amp for all further testing.

... I redid all of the measurements following your formatting with a new set of tungsol EL84's and here's what I got:

...
Cathode Current:  10.43v DC / 130.2Ω = 80.11mA 96.9mA

Drop Across 1kΩ:  (371v - 286v) / 1kΩ = ~85mA ---> 5mA for the preamp, 80mA for EL84 plates & screens.
(362v - 248v) / 1000Ω = 114mA

114mA overall minus 96.9mA for the output tubes ---> 17.1mA in the preamp.

There's no way your preamp should be pulling that much current.  AlNewman seems to have a point that the original can capacitor may be leaking current & pulling the supply low.

FWIW, my amp was worked on prior to my getting it.  The internal 3-section can was removed, and a terminal strip installed in its place.  I do have filter caps sprinkled throughout the preamp like your amp, but I don't have the old cap-sections being used as tie-points.  Perhaps evicting that old can is a first order of business.

I'm showing the measurements when the amp is biased closer to what you'd expect for a vintage vox. There's obviously something wrong with it elsewhere, hence the thread in the first place :) I can't do any permanent changes to the amp until I hear back from the shop to see what they want to do. Once I get that green light it'll get recapped and hopefully that's the cure. Also a new rectifier just in case it's not helping since it's outputting 10vdc less than yours despite being at a higher wall voltage.

I'm not sure what the current draw difference is between a 12au7 and a 12at7, but it did have an AT7 in V2 when I did the measurements.

Could you share photo's of the inside of yours? Would be cool to see!

2nd Filter Cap (after 1kΩ):  286v DC 248vDC

these seem to indicate the drag on the PS

What I'm thinking as well, but that can cap was replaced with a Weber at some point so how likely is it "already" bad? I guess there's no way to really tell until I can swap it and remeasure

[Yeatzee]

You can measure the voltage drop across the 2 10k and the 22k dropping resistors on your little tubes, and maybe 1 or more is out to lunch.  They are 12au7, so they can see higher current than normal, but it doesn't look like they're designed that way.  If you get much more than a few mA through those resistors, then I would suspect the original caps.

Forgot to check this, I'll do that tonight.

[shooter]

you can do the same "checks" at the tubes cathode resistor
EX; 1.5vdc / 1.5K R = 1mA

[HotBluePlates]

Cathode Resistor:  130.2Ω 74.8Ω

IMO, this is an enormous mistake, since the "right value" for a pair of EL84 is closer to 130Ω.  I recommend you use the 135Ω resistor in the amp for all further testing.

I'm showing the measurements when the amp is biased closer to what you'd expect for a vintage vox. There's obviously something wrong with it elsewhere, hence the thread in the first place :)

Maybe you caught my point, or perhaps it was missed.

It's like you said, "I have low energy," then fix the issue with an ounce of cocaine.  It will seem to work, but perhaps that's the wrong way to go about things.  I'm saying that using the 75Ω cathode resistor (appropriate for 4x EL84) is giving nothing but a false-indication here (where a cathode resistor suitable for 2x EL84 should be used).

Could you share photo's of the inside of yours? Would be cool to see!

This is my amp.  The external can capacitor is still present, but nothing is attached to it.  There are 5x total radial filter caps inside the chassis, with three at the end of the circuit board taking the place of the internal can capacitor.

I forgot to mention earlier I leave the reverb driver/recovery tube out of the socket (because I'm not gonna use the onboard reverb anyway).  So I might need to install it and re-measure to give an apples-to-apples comparison.

[HotBluePlates]

... I'm saying that using the 75Ω cathode resistor (appropriate for 4x EL84) is giving nothing but a false-indication here (where a cathode resistor suitable for 2x EL84 should be used).

See the first 3 pages of this PDF for advice on setting cathode resistor values for output tubes.

AWV was effectively "RCA in Australia" so this amounts to the tube manufacturer describing how their product should be used.  Indeed, the articles seem to be written by the folks who are in the lab building new amp circuit from which the data sheet figures are taken (the 6V6 noted in the article was a new tube type released only a year or two earlier).

[Yeatzee]

Haven't abandoned this, just put to the side while it's worked out with the dealer. After some back and forth with their tech, they agreed they should have replaced the original caps... particularly since the listing said they did, and agreed to give me a refund that will at least cover a recap across the board + a new rectifier. Parts have arrived, so just need to find the time to dive back in. Hopefully the recap will reduce the voltage drop and get everything to bias happy with the stock value. I was working with my hands essentially tied behind my back while I waited on approval from the dealer so now gloves are off and we can really dig in. Excited!

[Yeatzee]

Well, got a lot of work done on the amp. Full electrolytics swap with an F&t can cap and then mod everywhere else plus a 120R bias resistor which is the closest I had to the original value.





And drumroll...... No change lol. I also tried a new JJ rectifier and the voltage was actually lower with that. Right around 263vdc plate to cathode voltage, about 9.5vdc across the resistor with 120vac from the wall. 80ish percent dissipation.

Disappointing, but we've taken care of a lot of possibilities so progress was made regardless! I didn't get a chance to play the amp yet, but it seemed impressively quiet and I'm sure I'll prefer the less filtering vs what was in it before. I am curious about trying different preamp tubes and seeing if that affects things. Any other ideas to try next?

[HotBluePlates]

... Full electrolytics swap with an F&t can cap and then mod everywhere else plus a 120R bias resistor which is the closest I had to the original value.

And drumroll...... No change lol. ...

What does the 1kΩ resistor between the first couple filter caps measure?

If 1kΩ, replace it with a 470Ω if you need higher supply voltage.

[AlNewman]

I'd measure the current at your dropping resistors in comparison with the cathode resistors at each node and see if the numbers add up.

[BrianS]

And remember, if the amp sounds good, then it is good.  You can tinker out of curiosity, but if it were my amp I would probably leave it as-is.

[Yeatzee]

Alright here's the latest!

Wall voltage: 120.5vac
Rectifier output: 378 vdc
2nd filter cap (after 1k resistor): 281
El84 screen pin 9: 281
El84 plate pin 7: 275
El84 cathode pin 3: 9.70
Cathode resistor: 119.4R
Cathode current: 9.70 / 119.4 = 81.2mA

Drop across 1k: (378 - 281) / 1017 = 95.3mA 

What wattage is that 1k dropping resistor/ I can't tell from the markings, and the schematic doesn't say.

[AlNewman]

Those voltages look good, comparable with HBP's.
So what did you change between those and the voltages you posted yesterday??

[Yeatzee]

Those voltages look good, comparable with HBP's.
So what did you change between those and the voltages you posted yesterday??

Absolutely nothing. Just a bit higher wall voltage I guess, amp settled in more, not sure. But we're still a ways off from HBP, his voltages are higher across the board and a good 3vac lower input and a larger cathode resistor. I'm definitely feeling better about the amp, but I do still think the voltages are low.

[AlNewman]

The 120R vs 130R cathode resistor could make up the few volts difference in the B+.  You're well within 10% of the schematic.

One thing noticeably different between yours and HBP's numbers is the draw on the preamp and PI, although if I measure current at all the cathodes on your original schematic, I'm getting around 12.5 mA, where yours is around 14mA.  If you changed the bypass caps, and unless you are getting DC leakage at your coupling caps, that difference could be a few plate or cathode resistors out of spec.

If you're worried about it, do the math and then you'll know.

[Yeatzee]

The 120R vs 130R cathode resistor could make up the few volts difference in the B+.  You're well within 10% of the schematic.

One thing noticeably different between yours and HBP's numbers is the draw on the preamp and PI, although if I measure current at all the cathodes on your original schematic, I'm getting around 12.5 mA, where yours is around 14mA.  If you changed the bypass caps, and unless you are getting DC leakage at your coupling caps, that difference could be a few plate or cathode resistors out of spec.

If you're worried about it, do the math and then you'll know.

The schematic would be based off 115vac from the wall no? So I might be within 10% for the noted voltages on the schematic, but that's at 120vac. HBP is at 117vac in his. I guess I've never seen how much of a difference a few volts from the wall makes, so I'm ignorantly assuming that difference in AC voltage is hiding the VDC differences a good bit. Something I can test at another time.


*removing my stupidity*

Current preamp tube compliment is AX7, AT7, AU7, AU7(PI)

[AlNewman]

Well, since the cathode bypass caps stop dc voltage, that means you absolutely can measure DC voltage across the resistors with the caps in place.  Assuming your bypass caps aren't leaky. 

[Yeatzee]

Well, since the cathode bypass caps stop dc voltage, that means you absolutely can measure DC voltage across the resistors with the caps in place.  Assuming your bypass caps aren't leaky.

I did try that night, but was not getting anything on the other side so perhaps I did something incorrectly. It was a late night. I do know the first set is tied to the trem circuit, it was bouncing all over the place.

[AlNewman]

Your schematic has voltages on it.  You should be able to measure at those points and compare.
You measured the cathode resistor on the power tubes, and that has a bypass cap.

[Yeatzee]

Like I said, I tried last night and was getting nothing on the other end. Thought I was missing something, just measured wrong. Late night, never measured for current before until this amp. Just confusing myself over something very simple.

[Yeatzee]

Could someone help explain to me how the tremolo works in this amp? The depth specifically, it does very little until about halfway and then just passed halfway it gets very deep and then from there it gets thumpy and all warbled / weird sounding. Never seen a depth control that worked like it. Original pot, measures just under 1meg. Not a whole lot going on so not sure why the behavior is that way.

[stratomaster]

Could someone help explain to me how the tremolo works in this amp? The depth specifically, it does very little until about halfway and then just passed halfway it gets very deep and then from there it gets thumpy and all warbled / weird sounding. Never seen a depth control that worked like it. Original pot, measures just under 1meg. Not a whole lot going on so not sure why the behavior is that way.

Looks almost identical in topology to the Princeton bias trem, except adapted for a cathode biased circuit.  What you're describing sounds like a pot taper issue.

As far as how it works, go read the valve wizard page on tremolo oscillators and come back with specific questions. I doubt you'll have any/many after getting through that write up.  The Intensity pot itself is just a voltage divider that sends the oscillator signal to the grids. You could also think of is as a blend pot that blends in the oscillator signal or pans between fixed grid reference and oscillating reference.

The LED biasing he mentions is definitely worthwhile, and elsewhere on this forum you'll see a trick where a diode across the outer lugs of the Intensity pot mitigates thump by cutting the useless part of the tremolo swing off.

You could try a J taper pot and a series resistor of say 150k-220k in between the pot and C21.  You'll find a similar resistor on the Princeton oscillator that is missing from this circuit.

[HotBluePlates]

One thing noticeably different between yours and HBP's numbers is the draw on the preamp and PI

... that's at 120vac. HBP is at 117vac in his. I guess I've never seen how much of a difference a few volts from the wall makes ...

Current preamp tube compliment is AX7, AT7, AU7, AU7(PI)

I don't recall if I mentioned earlier:  I did not even install V3 (12AU7) that is on either side of the reverb tank.  That's because I know I will never use the cheesy on-board reverb of the Domino/SRT.

Could someone help explain to me how the tremolo works in this amp? ...

Just like a Princeton Reverb:  output of the Depth pot is applied at the grids of the output tubes to modulate their bias.

... The depth specifically, it does very little until about halfway and then just passed halfway it gets very deep and then from there it gets thumpy and all warbled / weird sounding. ...

The Vox uses an audio taper Depth pot.
The Fender uses a linear taper Intensity pot.

Different-taper likely accounts for the different impact-for-rotation.

A strong second factor would be that the output tubes are EL84, that have a smaller bias voltage and therefore don't need as much trem-signal before their bias is overcome to shut the tube off.  Especially since the SRT seems to swipe the Ampeg Reverberocket circuit whole-cloth, and Ampeg was using 6V6 output tubes.


(I've got both a Domino Super Reverb and a 1962 Ampeg Reverberocket.  I didn't switched on & test either amp before replying.  But I also haven't tried sweeping the Depth pot of either amp to note weird behavior:  I know the trem works on each amp, that if I turn the Depth pot the trem intensity will change, and that some setting of the control will give me a usable sound.)

Well, since the cathode bypass caps stop dc voltage, that means you absolutely can measure DC voltage across the resistors with the caps in place.

I did try that night, but was not getting anything on the other side ...

Tube cathode is on one side of the cathode resistor.  Ground is on the other side of most cathode resistors.

Mostly you attach Black to the chassis, and probe the cathode with your meter's Red lead.  The exception is the split-load inverter's bias resistor, which has ~56kΩ between it & ground.

[Yeatzee]

One thing noticeably different between yours and HBP's numbers is the draw on the preamp and PI

... that's at 120vac. HBP is at 117vac in his. I guess I've never seen how much of a difference a few volts from the wall makes ...

Current preamp tube compliment is AX7, AT7, AU7, AU7(PI)

I don't recall if I mentioned earlier:  I did not even install V3 (12AU7) that is on either side of the reverb tank.  That's because I know I will never use the cheesy on-board reverb of the Domino/SRT.

Could someone help explain to me how the tremolo works in this amp? ...

Just like a Princeton Reverb:  output of the Depth pot is applied at the grids of the output tubes to modulate their bias.

... The depth specifically, it does very little until about halfway and then just passed halfway it gets very deep and then from there it gets thumpy and all warbled / weird sounding. ...

The Vox uses an audio taper Depth pot.
The Fender uses a linear taper Intensity pot.

Different-taper likely accounts for the different impact-for-rotation.

A strong second factor would be that the output tubes are EL84, that have a smaller bias voltage and therefore don't need as much trem-signal before their bias is overcome to shut the tube off.  Especially since the SRT seems to swipe the Ampeg Reverberocket circuit whole-cloth, and Ampeg was using 6V6 output tubes.


(I've got both a Domino Super Reverb and a 1962 Ampeg Reverberocket.  I didn't switched on & test either amp before replying.  But I also haven't tried sweeping the Depth pot of either amp to note weird behavior:  I know the trem works on each amp, that if I turn the Depth pot the trem intensity will change, and that some setting of the control will give me a usable sound.)

Well, since the cathode bypass caps stop dc voltage, that means you absolutely can measure DC voltage across the resistors with the caps in place.

I did try that night, but was not getting anything on the other side ...

Tube cathode is on one side of the cathode resistor.  Ground is on the other side of most cathode resistors.

Mostly you attach Black to the chassis, and probe the cathode with your meter's Red lead.  The exception is the split-load inverter's bias resistor, which has ~56kΩ between it & ground.

I actually like the reverb! It's almost more room like than springy, I miss it when it's off but don't really notice it when it's on sort of deal. I'm diving back into the amp shortly, I'll try the diode trick just out of curiosity. 90% of this is just curiosity, good learning opportunities for me. The amp seems pretty healthy so far, no weird noises gets plenty of drive, sounds loud with my initial play test. Excited! It's quite a bright amp as well, not what I was expecting. My AC10 Twin seems a good bit darker in comparison, even the 12ax7 channel. I'll have to side by side them tomorrow, but there's definitely a ton of top end on tap with the SRT.


Yes me being super dumb with the current measurements. It happens! 

[Yeatzee]

Alright some further updates. First, the diode trick for the Trem thump was interesting. It didn't really fix it, it made it slightly less extreme when clipped on but seemed to add thump where there wasn't any before on lesser settings. It also slowed the speed way down. Fun experiment regardless. I also measured the pot while turning it and indeed the first half goes from 1meg to 900k resistance, and then falls off a cliff as you get around 1 o'clock. It's not worth replacing an original pot for, just kind of interesting.

I did try removing the reverb tube, V3, and remeasured voltages. Indeed the bias got a bit hotter without it, closer to 88% dissipation iirc.

And last I decided to replace all of the remaining piher looking resistors with 1w CF to match the rest on the board.


Played the amp a good bit last night with the ox box. Was working great, reverb like I said I actually really liked. Put the amp down for a couple hours, came back to play it and the reverb had this nasty sounding rattle noise from the spring that wasn't there before. Welp, that sucks! Stretching the middle small spring holding the two individual long springs so that they sit tighter and higher made it go away thankfully.

I did also swap v2 and 3 with a 12ax7 for fun. Sounds great so far!



I also did a speaker out box setup like on my ac10 Twin so that I can easily utilize both impedance options.

[stratomaster]

Alright some further updates. First, the diode trick for the Trem thump was interesting. It didn't really fix it, it made it slightly less extreme when clipped on but seemed to add thump where there wasn't any before on lesser settings. It also slowed the speed way down. Fun experiment regardless. I also measured the pot while turning it and indeed the first half goes from 1meg to 900k resistance, and then falls off a cliff as you get around 1 o'clock. It's not worth replacing an original pot for, just kind of interesting.

I can't imagine why or how the diode would affect the time constant of the RC oscillator.  Did you orient the diode as shown?  That diode should just clip off the top of the swing.

The Princeton has a 1M - 0.022μF low pass filter for reducing intensity as speed increases, that same 1M also limits the maximum depth available through the 250k pot.  The low pass can be left off, but the limiting resistor would be worth a shot to give you more usable range to the pot.  You can even try using tapering resistors to keep the original pot if that's your proclivity.

[Yeatzee]

Alright some further updates. First, the diode trick for the Trem thump was interesting. It didn't really fix it, it made it slightly less extreme when clipped on but seemed to add thump where there wasn't any before on lesser settings. It also slowed the speed way down. Fun experiment regardless. I also measured the pot while turning it and indeed the first half goes from 1meg to 900k resistance, and then falls off a cliff as you get around 1 o'clock. It's not worth replacing an original pot for, just kind of interesting.

I can't imagine why or how the diode would affect the time constant of the RC oscillator.  Did you orient the diode as shown?  That diode should just clip off the top of the swing.

The Princeton has a 1M - 0.022μF low pass filter for reducing intensity as speed increases, that same 1M also limits the maximum depth available through the 250k pot.  The low pass can be left off, but the limiting resistor would be worth a shot to give you more usable range to the pot.  You can even try using tapering resistors to keep the original pot if that's your proclivity.

Yes. I'm not the only one that's reported that, several threads I saw discussing it said the same thing. Why, I don't know.. I defer to the much more experienced lot but it very clearly slowed the trem speed down.

[Yeatzee]

Realized I never did share the videos now that they're live :)

For future reference...

Pt. 1

i=C3N9by21zorSLEtn

Pt. 2

i=bOMyDWo3eDoAAfCS

[Merlin]

Could someone help explain to me how the tremolo works in this amp? The depth specifically, it does very little until about halfway and then just passed halfway it gets very deep and then from there it gets thumpy and all warbled / weird sounding.

It gets thumpy because Vox didn't botther to tune the trem range, they just gave you the entire range from zero to 'everything the LFO can deliver' on one pot. That's a massive amount of modulation for EL84s at the maximum depth setting, so things get thumpy. If you want to tune it, just add a series resistor between the coupling cap and the depth pot (try 470k to start?) until you get a 'normal' amount of trem at the max setting. This should help with the taper issue too.

The diode is not a great mod IMO because it loads the oscillator on one half the the swing; I'm not surprised it skews the oscillation frequency as a result.

[HotBluePlates]

The diode is not a great mod IMO because it loads the oscillator on one half the the swing; I'm not surprised it skews the oscillation frequency as a result.

I wonder what a frequency-stable oscillator would look like in this situation.  I can see someone adding zener diodes at the output tube grids to keep them from being driven much beyond 0v grid-to-cathode, but that would still create the loading effect you noted.

[Yeatzee]

Could someone help explain to me how the tremolo works in this amp? The depth specifically, it does very little until about halfway and then just passed halfway it gets very deep and then from there it gets thumpy and all warbled / weird sounding.

It gets thumpy because Vox didn't botther to tune the trem range, they just gave you the entire range from zero to 'everything the LFO can deliver' on one pot. That's a massive amount of modulation for EL84s at the maximum depth setting, so things get thumpy. If you want to tune it, just add a series resistor between the coupling cap and the depth pot (try 470k to start?) until you get a 'normal' amount of trem at the max setting. This should help with the taper issue too.

The diode is not a great mod IMO because it loads the oscillator on one half the the swing; I'm not surprised it skews the oscillation frequency as a result.

I'm down to give that a try next time the amp is on the work bench to see!

First little gig with the amp, worked great! No issues.

[Yeatzee]

Test

[Yeatzee]

Forum wouldn't let me post this so had to do a test post first for it to work for whatever reason.

Following up since I spent some more time on the Trem issue. Ended up needing a 2.2m resistor inbetween the depth pot and the .1uF cap to take care of the obnoxious thumping and give a usable sound across the range. YMMV.



I also ended up drilling out an existing hole in the chassis a bit bigger and extending that hole through the side insert to bolt them together and get some rigidity back to this amp. Just so flimsy the way these were built, particularly today after the adhesive they used has degraded. This made it much more solid.




And finally I replaced the 2200pf cap between the tone and volume pot since it was a replacement that I felt like might be making the amp sound a little shrill. I didn't have any replacements in the same value so I used a vishay mkt1813 4700pf I had on hand. So far so good, definitely changed the cut control so that it's seemingly more effective earlier in the range. I'd love to find a wima tpf 2200pf (if anyone knows where to look let me know) but for now I'm happy, the amp is very bright so this may be an "improvement" I prefer.

And finally putting out a bit more than 10w it seems, despite the low plate voltage (8ohm load).