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Old 2nd April 2008, 06:33 PM   #1
wicked1 is offline wicked1  United States
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Default Ampex 6v6 PP Modifications

Hi,
ive got a pair of these Ampex 612 or 620's, and would like to get them sounding a little better. (they're labeled as 620's, but match the 612 schematic)
Basically, it looks like they used to have a tone control, and someone has removed it, and sort of butchered the feedback circuit. I redrew the schematic as mine have been modified, but I can't find my drawing anywhere! I'll have to take a look at them tonight and redo it.

I'll attach the 'stock' schematic here.

The issue I'm having is that they are very high heavy.
also, there is much more going on in this schematic than any of the schematics I've seen posted here as being a "good" P.P. amp.

If anyone has a few minutes and is bored, could you look at this schematic, and tell me how I could modify these to sound better. I don't really want the tone control on them at all. I'm afraid I don't know enough about amps at this point in time to know what is going on w/ the feedback.
I did disconnect the nfb all together, and it sounded bad.

Other than being high heavy, I think they sound great! If I connect them to some bass heavy speakers, or add a powered sub to fill in the low end, they're perfect.

I guess I should add that they aren't in their original cases, and don't have the speakers they originally come with. I've built them into a nice modern looking chassis. At this point, there's no reason to keep the amps stock, for historic reasons.
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Old 3rd April 2008, 12:16 AM   #2
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That looks like a Mullard style circuit, with the tone control stuff inside the NFB loop. Bad sound after disconnecting the NFB circuitry is not surprising.

You did say simplify. How far are you willing to go? Could you live with 6 W. of O/P from triode mode, as opposed to 15 W. from pentode mode?
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Old 3rd April 2008, 04:34 AM   #3
wicked1 is offline wicked1  United States
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A complete rebuild is an option I was thinking about. As long as I can use the same tubes, and obviously transformers.
I only found a couple of 'hi-fi' 6v6 schematics, and they were completely different than this amp.
What's the purpose of the first tube in this schematic? Is that splitting the input to the upper and lower tubes? I ask, because most of the schematics I've seen do not have that stage.

I think I could live w/ 6 watts. It's a tough decision. I got a single ended triode amp, so do have the speakers for it, but more power would be nice........

A major simplification of the feedback circuit would be nice too.
At this point, however, I'm not sure which components are boosting the treble, and which the bass, and how to figure out how much feedback to apply.
Everything I've been reading so far has been about single ended amps w/ no feedback.
I've got to start over now to learn about P.P.

So, a straight forward answer to what I should do w/ these amps is certainly welcome.
If there's a website out there that goes into the different aspects of these amps, a link to it would help too. (a link besides the search button here )

What would be great is a site that breaks the circuit into its components and explains each section!
Actually, I have seen a site like that. I try to find it again.
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Old 3rd April 2008, 04:55 AM   #4
korey is offline korey  United States
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I don't know the heater ratings on your power transformer, but just for the fun of it, try using 6L6GC's instead of the 6V6's. It won't hurt anything to try them for about 10 minutes. It may tax your 6.3volt tap if used for more than a half an hour, and may get a little warm. I use 6L6's on my maggie amp but I'm also not using the tuner/preamp so my milliamps is about the same. 6L6's are the same pinout as the 6V6, just higher voltage ratings.
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Old 3rd April 2008, 07:18 AM   #5
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You have the basis of a nice El Cheapo variant...

The 1st thing i'd do is as below.... except got the triode straping resostors, it is just snip-snip.

There is a lot more you can do....

dave
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Old 3rd April 2008, 09:08 AM   #6
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That NFB loop is confusing, I agree. To remove the tone control, you could replace the feedback network with a straightforward resistor, i.e. replace C11, C12, C14, R9, R13, R14, R15, R16, R22 and S1 with a resistor that goes from the spkr terminal to the cathode of V1. Try a value of 15k for the feedback resistor. If the amp is too sensitive, you can reduce the resistor to taste and/or strap V1 as a triode (see later).

It's a bit more complicated than that, though, because the more feedback there is, the greater the risk of HF instability. There almost certainly needs to be a capacitor in parallel with the feedback resistor. A 'ball park' figure of 100pF to 330pF might be suitable, but its exact value depends on the characteristics of the output transformer and no rule of thumb can be used. A square wave generator and oscilloscope are normally used to test the HF stability, and the capacitor is selected to give a clean 10kHz square wave at the speaker, without overshoot/ringing or round-shouldered appearance.

To explain the purpose of the first stage: it is a voltage amplifier that provides most of the gain. It may prove to have too much gain for the requirement and it might work better strapped as a triode, i.e. connect the screen directly to the plate and remove C2 and R5.

The phase is split by the 12AU7. Its plate loads R17 and R18 are unequal (100k and 120k) to compensate for the fact that there is a resistor (R2) as a common cathode load instead of a constant current sink. However, the difference of 20% in plate resistor value is unusual; 10% is more common.

I realize you prefer to use the same tubes but the 12AU7 is not a good one to use in any audio application and a 12AT7 would be much better.

A website that might help you analyze the amp on a stage-by-stage basis is Max Robinson's 'Fun with Tubes'.
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Old 3rd April 2008, 04:39 PM   #7
wicked1 is offline wicked1  United States
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Thank you guys so much! I was thinking of trying some of the things you've suggested, but being new to all of this, I'm still sort of afraid to just jump in and start changing things, w/out some advice first. Now that people who know what they're doing say to give it a shot, I think I'll warm up the soldering iron .
It's nice that they are mono blocks, so I change one, and compare the two.

Ok, I've got one more really really basic question, and it might fill in the gaps enough for me to finally understand what is going on.
I tried searching for the effects of networks of resistors and capacitors. Wikipedia was some help, but..... not enough.

I sort of figured out the tone circuit, in that I understand that the capacitor will attenuate the highs, the resistor will attenuate the lows.

I was wondering what the resistor/capacitor's in parallel are doing, but Ray may have answered that for me. The resistor is just lowering the power, and the capacitor is filtering some of the highs. (is that right?) (and here I'm talking about in the feedback portion of the circuit)

Then what's going on at the plate of the input tube, where the B+ goes through a resistor paralleled w/ a capacitor+resistor?
Then again, on the cathode of the 6v6's, there's a resistor and capacitor in parallel connecting to ground.

Wikipedia's explination of resistors and capacitors in parallel simply explains "The parallel configuration is generally of less interest to us" then gives some math... it never says what it's used for! I suppose the math is all someone w/ some E.E. training would need to understand it, but unfortunately, that's not me.

I guess my problem is that I understand the big picture, but not the basic EE details.

Thanks a lot!!!!!
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Old 4th April 2008, 12:32 AM   #8
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A capacitor in parallel with a resistor can be used as a low pass filter. Consider the case where that sort of arrangement connects the grid of a triode to ground. As frequency rises, the capacitor's reactance decreases. Eventually, the cap. shorts the resistor out.

I suggested triode wiring, because it avoids the issue of a regulated screen grid B+ supply for pentode mode and the lower power ensures that loop NFB gives you good bass extension, without O/P trafo core saturation risk.

Superior performance from pentode mode O/P tubes really requires that screen grid B+ be regulated. More linear operation is achieved. In particular, IM distortion (NASTY) is reduced.

Too large an error correction signal in the deep bass, from loop NFB, can cause the magnetic headroom of the O/P trafo core to saturate. Between filtering infrasonic noise at the amp's I/P and limiting power O/P, saturation of the O/P trafo core will be avoided.

Dave Dlugos (Planet10) was kind enough to mention "El Cheapo". You could easily turn those monoblocks into "El Cheapo" variants. The complete "El Cheapo" saga is here. Here's a "deep" link to the schematic.
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Old 4th April 2008, 10:09 AM   #9
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Quote:
A capacitor in parallel with a resistor can be used as a low pass filter.
I think Eli means a high pass filter. The reactance (AC resistance) of a capacitor is lower at higher frequencies. The uses of a cap are many and varied. Caps are used, among other things, to block DC between stages, to apply smoothing in power supplies, to provide a low-impedance path to ground for audio signals in a cathode bias circuit, to provide frequency-dependant filtering in tone-control circuits, to provide delays, to set time constants, and many more.

The cap I suggested adding in parallel with the feedback resistor will have the effect of reducing the impedance of the feedback path, i.e. applying more negative feedback, at higher frequencies, to offset the peak in gain that usually occurs and can cause poor performance/instability at high frequency.

I strongly suggest that you visit Max's website and have a good poke around, to understand more about what's going on. Max is a retired lecturer in electronics and has made an effort to make his website educational in a 'user-friendly' non-academic way.

If you're interested in having a good book that explains many things for the tube hobbyist without too much mathematics, you could buy Morgan Jones's excellent Valve Amplifiers. You can buy it from Amazon.com
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Old 4th April 2008, 01:26 PM   #10
SY is offline SY  United States
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wicked, if your experience is limited, a complete rebuild is going to be frustrating. And the Mullard circuit is actually a very good one- in the Morgan Jones book mentioned by Ray (and I can't recommend that book highly enough- you need it), there's a complete analysis of the circuit and a good discussion of the strengths and weaknesses.

If it were my amp, I'd regulate the screens (use a Maida reg), change the 12AU7 to a 6GG7/6FQ7, put a CCS in the tail of the phase splitter, and make the two plate resistors of that stage equal, and return a feedback resistor to the EF86 cathode. At that point, you'll have a pretty fine amplifier. If this is gibberish, I assure you that once you're through the Jones book, this will seem simple and obvious.
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