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Post subject: Twin Reverb distorts low frequencies
Posted: Mon Nov 03, 2014 7:46 pm
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Hi there folks! Long-time, occasional listener, first time caller here. Hoping someone else has run into this issue, but so far my Googling has failed to reveal the answer. Here's the problem: '73 Twin Reverb (DWG 010182). Bass notes distort heavily, while the mid and high end sounds fine, although maybe not quite as loud as they should be. Don't have enough experience with Twins to know exactly how it should sound, but the customer brought it to me with this complaint, and I've confirmed with my own ears, and lots of 'scope probing.

Troubleshooting:
1. Happens on both channels. (this suggests problem is in PI or power amp section, but not necessarily)
2. Using two frequencies, 100Hz and 1kHz, I checked the signal levels at various points to see where the low frequencies started distorting. Actual clipping doesn't occur until the output (OT output). However, the 100Hz signal is boosted significantly, relative to the 1kHz signal, on pins 6 of V1b and V2b. By significantly, I'm talking around 14dB.
3. Voltages seem to be in the ballpark, for the most part.
4. Since V1B and V2B share a cathode resistor and bypass cap I thought maybe that was the culprit. The 820 Ohm resistor measured around 840, and removing the cap temporarily did lower the bass response, but not significantly.
5. New tubes all around made no difference.
6. Amp exhibits problem with various speakers. Early on, I also ran it into my Weber MASS attenuator to rule out speaker issues.
7. Cussing the amp made no improvement whatsoever.

Like I said earlier, I'm assuming it's something common to both channels, like the PI, but it's also possible that someone made a mod to both channels that caused this weirdness. But if that's the case, I haven't found it yet.

Any thoughts?

TIA!
Sam

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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Tue Nov 04, 2014 2:50 am
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Check Output Transformer.
Speakers


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Tue Nov 04, 2014 3:15 am
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" However, the 100Hz signal is boosted significantly, relative to the 1kHz signal, on pins 6 of V1b and V2b. By significantly, I'm talking around 14dB. "

This kinda sounds like you have a leaky coupling cap(s) in this part of the circuit (V1 and V2). Check the VDC after each coupling cap between stages. You should not see more than a few mV on pins 2 or 7 of the 12AX7 (V1 of V2). Or on grids of the 12AT7 phase inverter.

You may also have some issues with the tone stacks. Check those resistors and caps in that part of the circuit. Also, look for problems with the cathode tied resistor and bypass caps --- on the 12AX7's

I'd rule these out before going to the output stage. Bass heavy issues are usually within the gain stage (pre-amp stage) & tone stages.


Good luck! Keep us posted.



http://ampwares.com/schematics/100W_Mas ... in_Rev.pdf


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Tue Nov 04, 2014 3:25 am
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BMW2002Ti wrote:
" However, the 100Hz signal is boosted significantly, relative to the 1kHz signal, on pins 6 of V1b and V2b. By significantly, I'm talking around 14dB. "

This kinda sounds like you have a leaky coupling cap(s) in this part of the circuit (V1 and V2). Check the VDC after each coupling cap between stages. You should not see more than a few mV on pins 2 or 7 of the 12AX7 (V1 of V2). Or on grids of the 12AT7 phase inverter.

You may also have some issues with the tone stacks. Check those resistors and caps in that part of the circuit. Also, look for problems with the cathode tied resistor and bypass caps --- on the 12AX7's

I'd rule these out before going to the output stage. Bass heavy issues are usually within the gain stage (pre-amp stage) & tone stages.




Good luck! Keep us posted.



http://ampwares.com/schematics/100W_Mas ... in_Rev.pdf


+1


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Tue Nov 04, 2014 6:19 am
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stratele52 wrote:
Check Output Transformer.
Speakers


Thanks for that, Stratele52! I was starting to suspect the OT might be the problem because even though the bass-heaviness starts after the tone stacks, it doesn't change the fact that clipping is occuring on the output. And from everything I've always heard, a Twin won't (or shouldn't) clip, so tranny saturation was about the only thing left that I could think of.

BTW, a few other things I didn't mention before:
1. FWIW the output power measured around 77W before clipping with a 1kHz signal into a 4 Ohm dummy load. Can't say for sure, but I think the power was the same with a 100Hz signal. Need to double check that.

2. The amp is in excellent condition. Not mint, but very, very clean, with no rust or other typical signs of aging you'd expect from an amp that's 41 years old. The point being that the tranny isn't rusted, so I wouldn't suspect shorted laminations.

3. The PS caps are original, but they look good, with no signs of leaking or bulging.

4. Modded the hum balance control to a bias pot last night after posting this.

5. The guts (wiring, solder joints, etc) appear to be as per factory, so no real signs of someone screwing around with the circuit, except for a line out jack with a pot on it. Hmmm, now that I mention that, it seems like a good place to focus.

And finally, what's the best way to test the tranny for this kind of issue? Again, as clean as it looks, and given that the output power is in the ballpark, there's nothing obvious or catastrophically wrong with the tranny, so I'm not sure how to test it.

Thanks again!
Sam

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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Tue Nov 04, 2014 6:37 am
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BMW2002Ti wrote:
" However, the 100Hz signal is boosted significantly, relative to the 1kHz signal, on pins 6 of V1b and V2b. By significantly, I'm talking around 14dB. "

This kinda sounds like you have a leaky coupling cap(s) in this part of the circuit (V1 and V2). Check the VDC after each coupling cap between stages. You should not see more than a few mV on pins 2 or 7 of the 12AX7 (V1 of V2). Or on grids of the 12AT7 phase inverter.

You may also have some issues with the tone stacks. Check those resistors and caps in that part of the circuit. Also, look for problems with the cathode tied resistor and bypass caps --- on the 12AX7's

I'd rule these out before going to the output stage. Bass heavy issues are usually within the gain stage (pre-amp stage) & tone stages.


Good luck! Keep us posted.



http://ampwares.com/schematics/100W_Mas ... in_Rev.pdf



Hey BMW2002Ti, thanks for the suggestions! I'll check the DC levels as suggested. But as mentioned in the original post, I checked the 820 Ohm cathode resistor and 25uF bypass cap that is shared by V1B and V2B. The resistor was close enough (840), and removing the cap made a slight difference, as expected, but nothing major.

IDK if I'm placing too much importance on this, but the fact that it is NOT clipping in the preamp or PI sections makes me think it has to be in the OT. Granted, it is definitely bass-heavy after the tone stack, but I would think the power amp section could handle however much bass gets thrown at it, and cleanly amplify that signal, as long as the signal feeding the power tubes isn't clipping. Make sense?

As I type this I'm seeing your suggestion of leaking coupling caps being the best place to start. Especially on the PI outputs (V6A/B, pins 1 & 6). If there was leakage there it could cause saturation on the OT, right?

Alright, time to break out the DMM and fire up the 'scope!

Thanks!
Sam

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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Tue Nov 04, 2014 7:28 am
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swellsound wrote:
[
And finally, what's the best way to test the tranny for this kind of issue? Again, as clean as it looks, and given that the output power is in the ballpark, there's nothing obvious or catastrophically wrong with the tranny, so I'm not sure how to test it.

Thanks again!
Sam



Testing Output Transformer

1- Signal generator set to 1000 hertz AC 0.5 volts

2- Send this signal to OT secondary , check with accurate meter to have 0.5 VAC on OT tap. Or close

3- Read AC voltage at OT primary

4- For exemple if you read 14.35 VAC you divide by 0.5 ( or what you read at 1- )

14.35 / 0.5 = 28.7

5- Square the result ; 28.7 X 28.7 = 829.67

6- Multiply by secondary output impedance where you are connected , say 8 ohms
829.69 X 8 = 6631

7 - This reading 6631 is the primary input impedance

If you use two 6L6 in push pull , look on Tube Book for Plate Load resistance . You’ll see you that you need 6000 to 3800 ohms for two 6L6 in PP class AB

The reading we got 6631 is on spec
_______________________________________
If transformer is no good your reading will be very far from what we should get .

By experience these test do not work on Super rever Transformer or other with 2 ohms Output Transformer impedance

You can do this test on unknow OT to see how / where you can use it


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Tue Nov 04, 2014 8:17 am
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stratele52 wrote:
swellsound wrote:
[
And finally, what's the best way to test the tranny for this kind of issue? Again, as clean as it looks, and given that the output power is in the ballpark, there's nothing obvious or catastrophically wrong with the tranny, so I'm not sure how to test it.

Thanks again!
Sam



Testing Output Transformer...



Thanks for the refresher, Stratele52! It's been a long time since I tested a tranny. But since my last post I did some more poking around and here's what I found:

1. The clipping is occuring on the plates of the output tubes! It's actually not as pronounced on the OT secondary.
2. Plate resistors all seem to be in spec.
3. Removing the line out circuit that had been added made no improvement.
4. Resistance readings on OT primary were as follows:
Blu-to-Red measured 37 Ohms, Brn-to-Red was 35 Ohms.
5. PS resistors in spec, but the voltages are a bit low. The PI plate voltages were about 30V lower than spec, and the output tube plate voltages were also about 30V under spec. Interestingly, the preamp plate voltages are very close to spec.

Does this sound about right? Does this point to anything different, or in your opinion should I still focus on the OT?

Thanks!
Sam

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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Tue Nov 04, 2014 8:54 am
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I don't think it's the output tranny. What is the idle bias current on those 6L6GC's?

Lower plate voltage on those 6L6GC's could mean a tired PSU. How old are those main filter caps?


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Tue Nov 04, 2014 9:04 am
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BMW2002Ti wrote:
I don't think it's the output tranny. What is the idle bias current on those 6L6GC's?

Lower plate voltage on those 6L6GC's could mean a tired PSU. How old are those main filter caps?



+1


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Tue Nov 04, 2014 9:16 am
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I , sometimes see bad resistor on Phase Inverter and voltage was right .

About your amp ; Reading some clipping and frequency to see where trouble come from it is not easy to me .

As say BMW check Power Supply voltage ans I say when playing , how it drop .......

Can you try with other speakers ?

About OT , it is so easy to test .

On old amp like that I would replace all bypass caps, those on the 12XX7 tubes plates and filter caps too as BMW say . This help ( and more ) to "tune" the amp .


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Tue Nov 04, 2014 3:54 pm
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BMW2002Ti wrote:
I don't think it's the output tranny. What is the idle bias current on those 6L6GC's?

Lower plate voltage on those 6L6GC's could mean a tired PSU. How old are those main filter caps?



I set the bias at 35mA.
Tried numerous speakers.

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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Tue Nov 04, 2014 4:00 pm
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swellsound wrote:
BMW2002Ti wrote:
I don't think it's the output tranny. What is the idle bias current on those 6L6GC's?

Lower plate voltage on those 6L6GC's could mean a tired PSU. How old are those main filter caps?



I set the bias at 35mA.
Tried numerous speakers.



For wich plate voltage ?

35 ma is good for not more than 380 volts Plate voltage


http://www.webervst.com/tubes1/calcbias.htm


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Wed Nov 05, 2014 6:18 am
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Hi Stratele52 and BMW2002Ti. Thanks to both of you for the suggestions. Haven't had a chance to test the OT but I have a little more info.

1. The PS caps are all original. Yellow Mallorys. FWIW, physically they appear to be fine.
2. Tested the PS sag with a 100Hz sine wave into 4 Ohm load.
Results:
Output Voltage V7-10 Plate voltage (VDC)
0 (no signal) 453
5Vrms 447
10Vrms 439
15Vrms 427
*Banging chords on a guitar ~400

*When playing chords fairly hard the plate voltage drops to around 400V. This seems like a pretty significant drop. But I don't have enough experience with Twins, or this type of test, to know if the PS should be stiffer than that, but I'm guessing it should?

Re: Bias current: I'm a bit confused Stratele52. You say a bias current of 35mA is ok for no more than ~380V plate voltage, and you reference the Weber bias calc. But I had actually used the Weber bias calc to come up with the 35mA value. I thought this was a very conservative (cool) value for the 450ish plate voltage. This is close to 50% dissipation (~15W) at idle. I also experiemented with hotter bias settings (into the high 40s), but the problem persists.

Thanks!
Sam

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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Wed Nov 05, 2014 6:47 am
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Bias ; wich tubes ? 6L6 or 6L6 GC not same

Use bias calculator ; 35 ma at 380 v is for 6L6 at 70% plate dissipation . Most Fender amps if not all do not sound better at more than 50 to 60 % plate dissipation.

B+ voltage of 453 volts and if it drop to 400 when playing ; look filter caps are weak , need new ones.


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