6G9-B Tremolux (Brownface) voltage check - running too hot

[mlannoo]

My '62 Fender Tremolux was accidentally dropped a while back and since then it still works but after a half hour or so of use the chassis is EXTREMELY hot. So hot in fact, touching the metal sleeve of the speaker cable on the back would easily burn me.

The closest schematic I can find for this amp shows +365v on pin 8 of the GZ34 rectifier ... but when I check DC voltage I get 430v on pin 8. And both the power tubes and rectifier tube are scortching hot. Is this normal? I get the feeling something is wrong but I don't really know what to look for or what to check.

Any help would be greatly apprecitated. Again, the amp works fine, it just gets really really hot ... to the point that something seems wrong.

Here are voltages:

5.45v AC yellow wires from PT to rectifier across pin 2 and pin 8

429v DC at the CT of the OT (red wire)

Plate Voltage on preamps:
V1 pin 1 - 187v
pin 6 - 205v

V2 pin 1 - 187v
pin 6 - 207v

V3 pin 1 - 235v
pin 6 - 236v

V4 pin 1 - 203v
pin 6 - 185v

Bias Voltage 6L6GC power tubes:
V6 pin 5 - -37.2v
V7 pin 5 - -37.3v

Plate Voltage on power tubes:
V6 pin 3 - 425v
V7 pin 3 - 425v

[tubeboy]

My '62 Fender Tremolux was accidentally dropped a while back and since then it still works but after a half hour or so of use the chassis is EXTREMELY hot. So hot in fact, touching the metal sleeve of the speaker cable on the back would easily burn me.

The closest schematic I can find for this amp shows +365v on pin 8 of the GZ34 rectifier ... but when I check DC voltage I get 430v on pin 8. And both the power tubes and rectifier tube are scortching hot. Is this normal? I get the feeling something is wrong but I don't really know what to look for or what to check.

Any help would be greatly apprecitated. Again, the amp works fine, it just gets really really hot ... to the point that something seems wrong.

Here are voltages:

5.45v AC yellow wires from PT to rectifier across pin 2 and pin 8

429v DC at the CT of the OT (red wire)

Plate Voltage on preamps:
V1 pin 1 - 187v
pin 6 - 205v

V2 pin 1 - 187v
pin 6 - 207v

V3 pin 1 - 235v
pin 6 - 236v

V4 pin 1 - 203v
pin 6 - 185v

Bias Voltage 6L6GC power tubes:
V6 pin 5 - -37.2v
V7 pin 5 - -37.3v

Plate Voltage on power tubes:
V6 pin 3 - 425v
V7 pin 3 - 425v

I’m not surprised to hear your tubes are running hot. At 429 Volts B+ and -37.2 V bias your static dissipation is approximately 33W per tube. Way to hot for idle condition. My calculations suggest you need at least -42 V of bias at your current B+. It’s possible your bias voltage was changed by the fall and requires adjustment. I would guess your power tubes are near the end of useful life. The schematic you’re looking at was closer to accurate when the electric company supplied 110VAC. Out of curiosity, what are the primary and secondary VAC values?

Chris

[bob p]

bias calculator:

http://www.webervst.com/tubes/calcbias.htm

[mlannoo]

I’m not surprised to hear your tubes are running hot. At 429 Volts B+ and -37.2 V bias your static dissipation is approximately 33W per tube. Way to hot for idle condition. My calculations suggest you need at least -42 V of bias at your current B+. It’s possible your bias voltage was changed by the fall and requires adjustment. I would guess your power tubes are near the end of useful life. The schematic you’re looking at was closer to accurate when the electric company supplied 110VAC. Out of curiosity, what are the primary and secondary VAC values?

Chris

Thanks for the reply. There is no bias adjustment pot in this amp so i'm not sure how to set the bias correctly. I understand how it is done (with a resistor) but not exactly how to calculate the value needed.

secondly, to be honest I don't understand your question about the primary and secondary VAC values. I have limited experience.

thanks.

[tubeboy]

Thanks for the reply. There is no bias adjustment pot in this amp so i'm not sure how to set the bias correctly. I understand how it is done (with a resistor) but not exactly how to calculate the value needed.

secondly, to be honest I don't understand your question about the primary and secondary VAC values. I have limited experience.

thanks.

My apologies, I didn’t look at the schematic. Because of the good details you furnished, I assumed that you were familiar with Volts AC “VAC”. Now that you mentioned the non-adjustable bias I’m left to reconsider my post. An increase in secondary voltage would also increase your bias supply voltage. You may have a problem in the fixed bias circuit but it would be easier to solve if you can safely measure the transformer secondary AC voltage to ground on each side. If your not sure how, please don’t try it.

Chris

[Bruce / Mission Amps]

Chris, how did you determnine the dissipation of each power tube from the parameters provided by mlannoo?

[mlannoo]

Thought I would share this. Help from a friend ... it explains a lot. Btw, I do have significant drift in several carbon comp resistors.

Well....the V's are a bit HIGH but that can be due to several factors, all explainable and still all OK.

1) Newer PT, some of the windings may be a hair off from the original.
2) Remember, the schematic was made up when wall voltage was 110V, now it's about 10% higher so today's V's are expected to be higher
3) Remember a GZ34 raises V level another 17V or so on the output. Although the original schematic calls for a GZ34.
4) Original V dropping resistors have drifted and are off, allowing more V through. Typically early 60s resistors are off up to 10% and more in some amps depending how much heat they've had.
5) Things being warm are fairly normal figuring transformer, rectifer tube and power tube heat

The issues here are all easily explained. If concerned he can:

1) Put in a 5U4 rectifier to lower voltages
2) Check every voltage dropping resistor to see how it's drifted.
3) Typically, repro trannys have a tendency to raise the voltages slightly from originals. That's been my experience. Usually that's OK.

Heat in a Fender amp is pretty normal. It could be he's watching the amp since he dropped it and never paid attention before. Now that he's looking at it it's warm....it probably always was. I can look at it if necessary, but I would doubt there is anything wrong. Pop in a 5U4 and that ought to drop things a bit more like the original schematic. I just don't see things too far off. Have to see it from there to tell more. I wouldn't be concerned at all unless it starts to SOUND bad, otherwise I'd leave it. Warm amps are normal, I can't even go near the back of my amps after a gig they are so hot.

[MWJB]

The voltages you have posted do not necessarily indicate a problem. Ideally you still need to assertain what your bias current is by measurement. Bias probes would be a good idea, Weber do some right-angled bias rite heads specifically for the smaller tolex head amps. If you get more than 40mA per tube you need to increase the value of the 33K resistor, in parallel with the electrolytic cap on the small bias circuit board, next to the pilot light.

Your amp probably got just as hot before the fall. A degree of heat, as your post states, is normal.

[mlannoo]

The voltages you have posted do not necessarily indicate a problem. Ideally you still need to assertain what your bias current is by measurement. Bias probes would be a good idea, Weber do some right-angled bias rite heads specifically for the smaller tolex head amps. If you get more than 40mA per tube you need to increase the value of the 33K resistor, in parallel with the electrolytic cap on the small bias circuit board, next to the pilot light.

Your amp probably got just as hot before the fall. A degree of heat, as your post states, is normal.

i agree! it probably ran that hot before the fall ... i'm going to do a cap job and replace the resistors that have drifted anyway. i have a Weber BiasRite and will check the bias when i install the new power tubes. Thanks for the feedback.

[tubeboy]

Chris, how did you determnine the dissipation of each power tube from the parameters provided by mlannoo?

Hi Bruce, I simply referenced a triode connection tube curve and estimated plate current to be around 72 to 74 mA at a plate voltage of 425 and grid of -37V. I don’t have enough information to calculate the actual plate power. This is a very coarse estimate and assumes a nominal 6L6GC. I’m still curious why the bias voltage isn’t running higher than spec as the B+ is. The B+ is running 17% above spec while the bias voltage increased by less than 4%. Any ideas?

Chris

[Jag]

Wow! I was too embarrassed to ask that!

Thanks for asking Bruce!

[Bruce / Mission Amps]

If you don't have a bias tester....
with the amp off and capacitors discharged, measure the DC resistances of the output tranny from the center tap to lugs 3 of the power tube.
When you have that in ohms, write it down.
Now set your voltmeter to read some high Dc voltage and with the amp turned back ojn and in play mode and running for a few minutes, CAREFULLY measure the actual voltage from the center tap of the OT to each lug 3 of the power tubes. CAREFULL here.
Now divide that DC voltage by the DC resistance you measured.
As an example... and this is only an example of how to do the math:
4vdc divided by say, 75 ohms = .053a, or 53ma.
So that would mean, that tube is drawing 53ma of idle current.
Now multiply that ma of current times your late voltage and you will have an
idea what the tube is dissipating at idle.
If you have 425vdc and the current is ,053a then... 425 x .053 = +22.5 watts.
In these amps I would shoot for around 17-18 watts at idle
Now remove the driver tube, the 12AX7 next to the power tube.
Remeasure your power tube idle current.
Do this one at a time with all the preamp tubes... one at a time with only one tube out of the socket at a time.
If the actual idle current changes a lot, then your "dropping the amp" has created some kind of ultrasonic oscillation in the amp that has the idle current screaming with AC current mixed in.
If it doesn't change anything, then I bet the amp's power tubes already were running hot or you have a bad power tube or you have discovered your bias supply filter cap is worn out and your bias voltage is just too low now and you just noticed it being sensitive to possibly damaging the amp after the fall.

[Enzo]

My first thought when I saw the bias about dead on but the B+ about 60 volts too high was: wrong power transformer. WHat number transformer is in there?

[MWJB]

Could the lack of a big swing in the bias voltage simply be down to the fact that the bias supply is derived from the rectifier AC winding, via a 100K dropping resistor, and that you would need a significant swing in voltage or current draw to get a big change here?

Also, I'm not overly confident in the schematic figures...330-0-330VAC into a GZ34 and only 365dc out? But as Enzo's says, a quick check of the PT number would be useful.

[mlannoo]

Thanks Bruce! - for the detailed instructions. I will try that and report my findings. I will also double check the number on the replaced Mojo PT.

Glad to find there are still helpful people out there! ... since the demise of the 18watt.com forum.

One thing I’m failing to understand is how the bias is set on this particular amp which has no bias control pot…. Well, maybe not HOW but WHERE? I thought the bias was determined (or created) by a resistor between the cathode of the power tubes and ground …. At least that’s the way I understand things to work in the EL84 36w clone I recently built. I did come across some mention of a “bias supply circuit” for this amp which supposedly is the 33K resistor in parallel with a 25uf/50v cap on a separate board up by the pilot lamp …. That is confusing me because I’m not understanding the connection between that and the power tubes …. I see it’s connection to the GZ34 rectifier.

FYI .. although it’s not shown in the original layout for this amp, I do have the addition of a resistor on pin 3 of both power tubes that appear to be not original and both are shrink wrapped so I can’t identify the values. I was originally assuming that those were the resistors that set the bias - but i guess not because pin 3 is the plate correct? so why the addition of a resistor to the plate?

http://www.ampwares.com/ffg/schem/tr...9-b_layout.gif