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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Wed Nov 05, 2014 8:15 am
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" 1. The PS caps are all original. Yellow Mallorys. FWIW, physically they appear to be fine. "

Those caps are over 40 years old. You need to replace them. Do this first, then measure all new voltages and idle bias. You sometimes can't tell the viability of parts from looks. Don't even bother to static test them. They are prolly failing under load.

I doubt those caps have much filtering or energy reserve left in them. Electrolytic caps, esp paper type, lose their electrolytic fluid to evaporation. To a point of needed replacement in 30 years. Regardless, if the amp was used of not.

My bet, is that your low frequency distortion and clipping is prolly due to poor power reserve from the original Mallory caps.

I like F&T caps. For the TR, used the 80mfd/450VDC caps for the first stage (2 needed). I think that the extra voltage rating is more important considering today's outlet voltages and the extra life of higher VDC ratings --- than total filtering capacitance.

Two 80mfd/450VDC = 40mfd total (but 900VDC rating) versus two 100mfd/350VDC = 50mfd OEM total 700VDC rating). 10mfd filtering difference, in a stage of the PSU, usually makes little difference to overall performance or life of the amp. Just my opinion.

And the 22nfd/500VDC for the preamp/effects stages of the PSU.

-----

https://www.tubesandmore.com/products/C-ET80-450-FT


https://www.tubesandmore.com/products/C-ET22-500-FT


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Wed Nov 05, 2014 9:00 am
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BMW2002Ti wrote:
" 1. The PS caps are all original. Yellow Mallorys. FWIW, physically they appear to be fine. "

Those caps are over 40 years old. You need to replace them. Do this first, then measure all new voltages and idle bias. You sometimes can't tell the viability of parts from looks. Don't even bother to static test them. They are prolly failing under load.

I doubt those caps have much filtering or energy reserve left in them. Electrolytic caps, esp paper type, lose their electrolytic fluid to evaporation. To a point of needed replacement in 30 years. Regardless, if the amp was used of not.

My bet, is that your low frequency distortion and clipping is prolly due to poor power reserve from the original Mallory caps.

I like F&T caps. For the TR, used the 80mfd/450VDC caps for the first stage (2 needed). I think that the extra voltage rating is more important considering today's outlet voltages and the extra life of higher VDC ratings --- than total filtering capacitance.

Two 80mfd/450VDC = 40mfd total (but 900VDC rating) versus two 100mfd/350VDC = 50mfd OEM total 700VDC rating). 10mfd filtering difference, in a stage of the PSU, usually makes little difference to overall performance or life of the amp. Just my opinion.

And the 22nfd/500VDC for the preamp/effects stages of the PSU.

-----

https://www.tubesandmore.com/products/C-ET80-450-FT


https://www.tubesandmore.com/products/C-ET22-500-FT


+1000


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Thu Nov 06, 2014 8:04 pm
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Aspiring Musician
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Quote:
7. Cussing the amp made no improvement whatsoever.

That made me laugh. :lol:
Quote:
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.

Later, you wrote:
Quote:
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.


I would think this a fairly obvious mod, unless it's not a mod.

At this point, I'm wondering if you have a '73 twin reverb(100 W), or a later version.(135W)
Does this amp have a pull boost switch on the master volume control? Other late 70's changes included a line out jack, hum balance pot, and 135 Watt output.

Some clear pictures would help the experts here offer more focused advice.

" 1. The PS caps are all original. Yellow Mallorys. FWIW, physically they appear to be fine. "

Quote:
BMW2002tii wrote:

Those caps are over 40 years old. You need to replace them. Do this first, then measure all new voltages and idle bias. You sometimes can't tell the viability of parts from looks. Don't even bother to static test them. They are prolly failing under load.

I doubt those caps have much filtering or energy reserve left in them. Electrolytic caps, esp paper type, lose their electrolytic fluid to evaporation. To a point of needed replacement in 30 years. Regardless, if the amp was used of not.

My bet, is that your low frequency distortion and clipping is prolly due to poor power reserve from the original Mallory caps.

I like F&T caps. For the TR, used the 80mfd/450VDC caps for the first stage (2 needed). I think that the extra voltage rating is more important considering today's outlet voltages and the extra life of higher VDC ratings --- than total filtering capacitance.

Two 80mfd/450VDC = 40mfd total (but 900VDC rating) versus two 100mfd/350VDC = 50mfd OEM total 700VDC rating). 10mfd filtering difference, in a stage of the PSU, usually makes little difference to overall performance or life of the amp. Just my opinion.

And the 22nfd/500VDC for the preamp/effects stages of the PSU.


Quote:
Stratele52wrote:
+1000


Have to agree with BMW2002tii and Stratele52 here. I'd also replace the cathode bypass and bias supply caps as well. (Pay attention to the polarity on the bias supply cap.)

Retroverbial would have posted some educated, eloquent, erudite reply here, but your electrolytic caps fluid have probably turned into a congealed, crystallized, conglomerated mess.
(Sorry, the alliteration mood strikes me once in awhile.)

vinyl

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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Fri Nov 07, 2014 2:04 am
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vinyl wrote:
[

Have to agree with BMW2002tii and Stratele52 here. I'd also replace the cathode bypass and bias supply caps as well. (Pay attention to the polarity on the bias supply cap.)


vinyl



+1


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Sun Nov 16, 2014 9:19 pm
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Hi folks, sorry for ditching the thread for a bit, but I put it aside while I waited on parts, and cleared out some of the other repairs I had in the queue.

Before I go into detail about the latest results, I should point out that the voltages that I had previously reported as being ~30VDC low (including B+) were actually within a few volts of spec even before replacing the caps as mentioned below. Not sure why I was getting the low #s I reported, but I've checked the voltage levels numerous times since then and they are consistently in spec., with a few exceptions I'll mention in a minute.

Here's the latest:
1. Replaced the PS caps as follows: 2X 100uF/350VDC with Sprague Atoms that I had on hand. Two of the remaining three caps (20uF/500VDC) replaced with some Illinois Capacitor 22uF/450VDC caps I also had on hand, EXCEPT I used the one of the original yellow Mallorys in the spot adjacent to the two 100uF/350VDC caps. Reason being, that node actually has around 452VDC, so I thought it'd be better to put an old cap with a 500V rating in there than a new cap with a 450V rating. Actually, I tried it with one of the IC 22/450 caps first, but then changed it just to be safe, although given the two choices it was really a tossup. Anyway...

This improved the voltage sag to the point where B+ was dropping down to around 420VDC now, as opposed to ~400 with the original caps. GREAT! But not so fast... it still clipped and sounded like a fuzzbox when I hit it hard. I'm sure it had a little more headroom than before, but any improvement just wasn't enough. And the table I added before, where I recorded B+ vs. Output level, improved somewhat, but nothing like I expected. This time I got B+ #s of 450, 450, 442, 432, and 420VDC for Output levels of 0V, 5.0V, 10.0V, 15.0V, and 20.0V, respectively. And at 20.0V output, the amp was just below clipping. OK, now I have a dilemma. In the past I would have whipped out my abacus and done some hardcore number crunching to come to the realization that 20V into 4 Ohms (the Twin's speaker's impedance) = 100W. Hey, that's what the amp is spec'd at, so we're all good now! Time to button it up, and call the customer to come get his amp! But instead, I'm sitting here writing a dissertation that rivals War and Peace, trying to figure out WTH is going on here.

2. The question has been raised regrding which version circuit I have. As mentioned way, way, way back in time, the amp is the version in the CBS DWG NO. 010182, REV. F, from 11/17/71 (whoa, in a few minutes that will have been 46 years ago, to the day!) It has the push/pull boost, and hum balance pot, but I modded the pot to a bias pot. But otherwise, everything else matches the schematic, down to the voltages called out, with a few minor exceptions. The only two voltage discrepancies that seem worth mentioning -and I'm not sure they're truly worth talking about- are V1, pin 6 (schematic calls out 225, I measure 234 VDC), and V6, pin 1 (schematic says 320, I measure 308 VDC).

3. Swapped out the original cathode bypass caps with some nice Sprague 20uf/50V caps. No discernible improvement.

4. The output of the PI circuit clips if pushed hard enough, but the output of the transformer clips before the PI does.

Given everything that has been replaced, and the behavior of the circuit, I am back to thinking it must be the output transformer. Oh, and BTW, here's what I found when I tested the OT:

5: Output tranny tested with 1Vrms into secondary, measured 22Vrms out (performed at 100Hz and 1000Hz with negligible difference in results). 22x22=484, 484x4=1936 Ohms input impedance. Sounds about right, but IDK. I'm assuming that only a gross failure (shorted windings) of the tranny would show up in this test since it's at low voltage. Is it possible that the tranny tests ok at low voltage, but is failing at high voltage due to insulation resistance issues? Maybe it's been pushed hard and/or run without tubes enough to cause some arcing that only shows up at operating voltages?

6. Tubes are JJ 6L6GC. So the bias current of 35mA should be fine. BTW, I played around with the bias and the clipping does improve somewhat as I adjust it to "colder" settings, but you can see the crossover distortion by that time.

So, any thoughts? Does the output tranny sound like the most likely culprit, or does anyone have other ideas?

Thanks!
Sam

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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Sun Nov 16, 2014 11:07 pm
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First, change all the 20mfd/450VDC IC caps with new 20mfd/500VDC Sprague Atoms or better yet, 22mfd/500VDC F&T. Do not reuse that old Mallory.

The first cap after the two 100mfd/350VDC caps is the one that is directly connected to the phase inverter. If this one is inadequate, the tone will definitely be affected. Both channels.

All caps in the PSU are important for the overall tone of the amp.

Try that first and get back to us.


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Mon Nov 17, 2014 2:19 am
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BMW2002Ti wrote:
First, change all the 20mfd/450VDC IC caps with new 20mfd/500VDC Sprague Atoms or better yet, 22mfd/500VDC F&T. Do not reuse that old Mallory.

The first cap after the two 100mfd/350VDC caps is the one that is directly connected to the phase inverter. If this one is inadequate, the tone will definitely be affected. Both channels.

All caps in the PSU are important for the overall tone of the amp.

Try that first and get back to us.



+1000


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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Mon Nov 17, 2014 8:24 am
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BMW2002Ti wrote:
First, change all the 20mfd/450VDC IC caps with new 20mfd/500VDC Sprague Atoms or better yet, 22mfd/500VDC F&T. Do not reuse that old Mallory.

The first cap after the two 100mfd/350VDC caps is the one that is directly connected to the phase inverter. If this one is inadequate, the tone will definitely be affected. Both channels.

All caps in the PSU are important for the overall tone of the amp.

Try that first and get back to us.


After I signed off last night I realized I never finished detailing the PS cap replacement saga. When I mentioned that I put this job aside while I waited on parts, I meant to add that the parts I was waiting on were some F&T 22uF/500V caps to replace the old Mallory and IC caps. So the voltages I reported were with the two 100uF/350V Sprague Atoms, and three F&T 22uF/500V caps in place.

Anybody care to comment on the fact that the amp is putting out 100W before clipping? And that it does so @ both 100Hz and 1kHz? After sleeping on it this takes me back to my earlier supposition that the problem seems to be bass-heaviness in the preamp section(s), and not a fault of the OT since the amp produces 100W before clipping even at 100Hz. Would some waveform pics help? My main 'scope is an old Tektronix but I could snap a few pics of the screen if anyone thinks that would be enlightening.

Thanks!
Sam

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Post subject: Re: Twin Reverb distorts low frequencies
Posted: Mon Nov 17, 2014 9:29 am
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100 watts before clipping is what this amp should be producing (this is a Master Volume TR). Bass heaviness is usually a pre-amp section issue. Most likely, a leaky coupling cap(s) or too high plate voltage on one or more of the gain stage tubes (12AX7 or 12AT7).

A good test for the gain stage issue is to substitute a 12AY7 into each of the 12AX7 positions and listen to the resultant tone. If one position effects the bass response (in a good way) the most, that is prolly your culprit section. Check the voltages and coupling caps around this position.

For the 12AT7 positions, use a 12AU7. Do the same test. The idea is to insert a lower gain tube into each position one-by-one, and listen to the change.

Most likely, it's going to be either an input stage or the phase inverter. As these two have the most influence on overall tone. The PI has a long-tailed arrangement of resistors on the cathode. These resistors need to be in spec. If you rebuild this section, use good metal oxide, 1/2 watt, 5% or better tolerance resistors.

The input stage is very sensitive to plate voltages. You can either drop the voltage by changing the anode loading resistor values. Or use a lower gain tube (like the 12AY7) to compensate for the higher voltages. I would try the 5751, 12AT7, 12AY7 in that order, if you have those tubes to play with. If the bass problem goes away with another type tube, used this tube for awhile and see if this simple change solves the problem.

Measure DC voltages which pass the coupling caps (voltage on the next stage's grid). They should be small. Check voltages on V1, V2, V6 grids (pin 2 & 7) esp. They should be near or under 1 VDC. Anything higher may indicate a leaky coupling cap connected to that grid. Pull one leg of suspect cap and re-measure the voltage. If it drops to zero... you have your leaky cap. Replace all coupling cap with 600VDC rated ones. Same mfd.

Good luck! Keep us posted. :D


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