Vintage Gates program amps conversion

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beatnik

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Hi everyone

I have a pair of program amps from a vintage Gates Studioette console that I would like to convert for mic preamp / DI use.
I have attached the original schematics and it would be great if I can get some ideas and suggestions.

The first decision I need to make is how to modify the gain control.
So far I have wired the potentiometers like on the original schematic but would prefer having stepped gain control instead, for more precise stereo settings.
I am wondering wether I should be considering to implement a feedback based gain control, like on the Telefunken V76 for example.

Do you reckon it would be possible to implement a gain control with a simple 1 pole / 12 position switch ?
Are there particular advantages using a feedback gain control as opposed to the voltage divider approach ?
I guess the ideal position for the feedback control is between 1st and 2nd tube like on the Telefunken, but since here we also have feedback from the output transformer back into the 3rd tube, could that potentially be used as an additional gain control for adding harmonic distortion ?

Regarding the DI input, where you think is best to place that in the circuit, right after the input transformer or perhaps just in front of the output stage ? The REDDI di box has a very similar circuit with just a single tube with parallel connected triodes and it seems to have enough gain for DI use.

Thanks in advance







 

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rock soderstrom

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Before EMRR Doug will tell us what this amp is, my humble opinion on your questions.

Without knowing the transformers, this program amplifier has plenty of gain (>60dB) and can certainly serve as a microphone/Instrument amplifier.

You can split the coupled gain potentiometer into two seperate 100k log pots or stepped switches and adjust the gain and harmonic distortion of these two stages independently. This can be very cool soundwise, from clean to overdrive, everything is possible. Also for the DI input, which I would loop in between the transformer and the first gain stage. (Replace R17 with a 1M resistor). Maybe it would be useful to make the negative feedback (partially) switchable?  8)
 

emrr

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Notice R16 above the first pot, making a voltage divider?  It throws away all of the gain from the first stage, plus some.  Roughly -36dB.  You can lose R17 for mic use. 

There's 71dB gain even with that voltage divider.  The input transformer and first stage is 57dB gain by itself. 

The first 2 stages are max gain pentodes without feedback, only exception being if it's a version that does or does not have cathode bypass on the first stage.  I'm not a fan as it's hardly useful today, but needed with speech into low output mics for reporting.  I don't think it sounds great overdriven.  Depends on your tastes as a mangler operator.  You might consider making it a triode 1st stage to get initial gain somewhere more reasonable for modern use.  You'll still end up using an input pad for a lot of things, either way. 

In most of this series I've converted them to have one less stage, the second stage is the easy one to bypass, simply rewire the pot path.  That makes them like the one later standalone remote amp.  All of of the various console and multi-input remote amps are variations of this same circuit. 

Some of my numbers above may be off depending which version of the schematic you look at, but the basic ranges are correct. 

Precise stereo settings will be dependent not on the accuracy of your stepped controls, but on 1st or 1st/2nd tube gain, which will never be exactly the same, and likely drifts with warm-up.  You'll need a continuous trim somewhere if trying to be exact. 
 

beatnik

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Will definitely try the DI connected on the input transformer secondary.

Good point about R16, I wonder what was the reason behind that.

I was actually thinking about connecting the EF86 as triodes. That should help with microphonics as well. But the EF86 in pentode mode always sounded very nice.

I have another Gates amp, SA-134 I believe and that is 2x pentodes into a parallel connected 6SN7. The available gain is quite a lot. Perhaps that's the best way to go and that will also lower the heather current requirements.

You are right about the need of a trim control somewhere, it would be nearly impossible to have two perfectly matched channels.

I am somewhat intrigued by trying the gain control in the feedback path, a lot of great vintage amplifiers use this topology (OP-6, V76, Redd47,..)
Trying to do some research on the subject at the moment.
Generally speaking what are the pros / cons compared to the simpler voltage divider approach ?

 

emrr

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beatnik said:
Will definitely try the DI connected on the input transformer secondary.

Good point about R16, I wonder what was the reason behind that.

I was actually thinking about connecting the EF86 as triodes. That should help with microphonics as well. But the EF86 in pentode mode always sounded very nice.

I have another Gates amp, SA-134 I believe and that is 2x pentodes into a parallel connected 6SN7. The available gain is quite a lot. Perhaps that's the best way to go and that will also lower the heather current requirements.

I am somewhat intrigued by trying the gain control in the feedback path, a lot of great vintage amplifiers use this topology (OP-6, V76, Redd47,..)
Generally speaking what are the pros / cons compared to the simpler voltage divider approach ?

You change the harmonic content changing negative feedback by much. The WE 141A does that over a very wide range, it sounds quite different at each position.

The 134 is triode pentode triode. The rackmount version of that amp uses a good bit more NFB in the output stage to set a lower overall gain range. The console version does too, but ADDS a fourth stage before the gain control, so 2 stages before the gain control.

I wouldn't convert the last stage EF86 to triode. Pentode operation avoids Miller effect.

R16 isn't there in some versions.
 
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beatnik

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Having a variable distortion response is potentially interesting, ranging from clean tones to more coloured.

I will definitely add a pad on the output transformer secondary to allow overdriving the preamp.

You're right about the 134 the first 6J7 is wired triode. What are the other versions you are referring to ? Would be great to compare schematics
 

emrr

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This basic amp is used in every Gates console and remote amp in the same time frame, mid-50’s through mid/late ‘60’s.  Probably some schems in tech docs or various threads.
 
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