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Old 26th Mar 2023, 6:46 pm   #1
RogerLLL
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Default 6V6 ECC83 push-pull amp project

First of all many thanks to G4XWDJim for giving me this amp, along with 2x metal clad 6V6s! I have 2x ECC83 valves on order. In the meantime I've been reverse-engineering the amp to produce a schematic, as in the attached. Not an easy exercise, so I hope I've managed to do that accurately.
This is my first experience with a p&p amp so my understanding of the final 2 sections is limited. Seems to me that the first 2 stages with V1 a&b are reasonably straightforward, with the low gain input bypassing V1 and going straight into the third stage V2a. I notice there's no cathode bypass cap on this stage, is that because the NFB comes in here, or could one be added?
I'm less clear about what's happening with V2b, I'm assuming this is acting as a phase inverter for feeding into the 2x 6V6 power stage?
Jim did say that the 6V6s are biased low due to the limitations of the power transformer, so there could be scope for an upgrade there, but for now happy to use as-is.
Really not sure about the tone circuit, and I struggled to get accurate capacitor values due to access & visibility.
I've also attached some pics. There are no transformer markings except the 27747 on the OT. I've searched but not found anything matching that, so would be good to know more if anyone has any insight?
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6V6 ECC83 Schematic v1.pdf
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Old 26th Mar 2023, 9:10 pm   #2
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Default Re: 6V6 ECC83 push-pull amp project

V2b is acting as a concetina phase splitter, see here
http://www.r-type.org/articles/art-010p.htm

Peter
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Old 26th Mar 2023, 10:00 pm   #3
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Default Re: 6V6 ECC83 push-pull amp project

That o/p tx looks just like a Pye Telecom transformer from 1950s/mid 60s era, pre transistor.
The part number fits in exactly with their parts listing as well. I have an ht lt one starting with 2 and about 6 digits. The varnish is identical as well.


The only strange bit is, it is far too big for a receiver audio o/p tx for just a local loudspeaker. Push pull would not be used either in a rx. Maybe, just a single ended EL90?
Although, it would be no use for an audio amp, it does look the right size for an am tx modulation transformer for maybe a 25W am vhf tx, requiring at least 15w of audio to modulate it.

Here is a picture of what I mean, 2x 6v6GT modulation for a pa output stage. You can see the 2x 6V6GT next to it.
However, the secondary will be a few K ohm impedance for the tx output stage matching.
https://www.radioenthusiast.co.uk/ne...ipment/#News-9

A mystery indeed!
Rob
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Old 26th Mar 2023, 10:14 pm   #4
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Default Re: 6V6 ECC83 push-pull amp project

Fuse in the earth connection im assuming is a mistake in the diagram
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Old 26th Mar 2023, 10:57 pm   #5
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Default Re: 6V6 ECC83 push-pull amp project

Well, someone on Facebook sent me a Link to a homemade HF tx where that very same transformer number is use as a modulation transformer. So, no idea how it works in your audio amp. Out of curiosity, measure the loudspeaker winding resistance and let me know.

https://www.stationqrp.com/. The first part, an 8W am tx download shows the picture of your transformer. I wonder if it has a low impedance winding for a public address loudspeaker, assuming it was used in a mobile transceiver originally? That would make sense.
Rob
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Last edited by CambridgeWorks; 26th Mar 2023 at 11:04 pm. Reason: Last paragraphs added
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Old 26th Mar 2023, 11:56 pm   #6
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Default Re: 6V6 ECC83 push-pull amp project

The HT supply circuit is slightly different from the one you've drawn.

You show a 40uF capacitor in parallel with the 22k dropper resistor. In fact I suspect that the can holds a dual capacitor - perhaps two 40uF units. The caps' two negative terminals will be connected together inside the can and will then be connected to the can. This, in turn, will connect them to 0V. The two terminals connected by the 22k are the caps' positive terminals. So after the bridge the components along the HT rail are:

33ohm current limiting resistor
First 40uF capacitor to ground (conventionally called the 'reservoir' cap)
270k bleed resistor to ground (to safely discharge the caps when the power is switched off)
Connection to the output transformer centre-tap
22k HT dropper resistor
Second 40uF capacitor to ground (conventionally called the first 'smoothing' capacitor)
Connections to the 6V6 screen grids and V2b anode load resistor
10k HT dropper resistor
8uF capacitor to ground (second smoother)
Connections to all the other ECC83 anode load resistors

Cheers,

GJ
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Old 27th Mar 2023, 1:10 am   #7
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Default Re: 6V6 ECC83 push-pull amp project

Some modulation transformers used in Pye RT's also had a winding to enable them to be used for public address in emergency vehicles.

A while ago, I had a thread going about the mod transformer used in early Pye Rangers (using EL90's) but some circuits of other Pye RT's were posted there too
https://www.vintage-radio.net/forum/...d.php?t=196821

I'm pretty certain that some Pye 2M base stations had a pair of of 6V6's modulating QQV03-40 PA's.

B
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Last edited by Bazz4CQJ; 27th Mar 2023 at 1:17 am.
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Old 27th Mar 2023, 6:18 am   #8
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Default Re: 6V6 ECC83 push-pull amp project

The pot on J1 likely goes to 0V at the end that also goes to the tone network/stack, which is likely a Fender style tone stack variant or similar (see http://www.duncanamps.com/tsc/ and http://www.kldguitar.com/Navigation/...ack-Works.html).

The 10k input stopper and 1M grid leak are typical for a guitar input amp.

No bypass cap is used for a global feedback connection, as that 2k7 needs to see both the output signal being fed back, and the input signal coming to that stage.
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Old 27th Mar 2023, 9:07 am   #9
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Default Re: 6V6 ECC83 push-pull amp project

The modulator on business 2 way radio will be designed for communication quality, approx 300Hz to 3.4KHz. What the transformer quality will sound like, needs to be heard.
Ed Dinning may have some idea?
Rob
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Old 27th Mar 2023, 10:53 am   #10
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Default Re: 6V6 ECC83 push-pull amp project

Also no 6V6 cathode bias in the schematic, for cathode bias the 470k grid resistors should be connected to ground

Lawrence.
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Old 27th Mar 2023, 10:58 am   #11
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Default Re: 6V6 ECC83 push-pull amp project

Quote:
Originally Posted by robinshack View Post
The modulator on business 2 way radio will be designed for communication quality, approx 300Hz to 3.4KHz. What the transformer quality will sound like, needs to be heard.
Ed Dinning may have some idea?
Rob
You would expect that were the case, but...I have this vague recollection that the youthful G6Tanuki used the modulator in a Pye Vanguard as an amplifier to play "pop music of the day" and was very pleased with it. Perhaps he'll be along? I recall his comments as I still have a Vanguard chassis sitting in the garage. I'm sure he changed components in the AF pre-amp, but retained the original transformer.

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Old 27th Mar 2023, 11:12 am   #12
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Default Re: 6V6 ECC83 push-pull amp project

Some work has already gone into that amp already and I seem to recall that Jim said that it works. I assume you're going to try it 'as is' first once the valves arrive?

In case you have issues with the output transformer, I have a spare effectively new 8k to 4/8 ohms push pull opt suitable for your 6v6 output valves which you can have for cost of postage.

You will find the increase in volume quite remarkable. For a guitar amp you may want to cut down on the feedback and see it you prefer it that way (or use an on/off feedback switch).

Good luck

Gabriel
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Old 27th Mar 2023, 11:25 am   #13
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Default Re: 6V6 ECC83 push-pull amp project

Quote:
Originally Posted by G.Castle View Post
Fuse in the earth connection im assuming is a mistake in the diagram
I suspected I'd not represented that correctly! It's wired in an unusual way, one side of the fuse goes to ground, the other goes to a tap within the transformer. I'd normally expect a fuse to be inline with the live mains supply, so any insight as to how this works would be appreciated.
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Old 27th Mar 2023, 11:27 am   #14
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Default Re: 6V6 ECC83 push-pull amp project

Quote:
Originally Posted by robinshack View Post
Well, someone on Facebook sent me a Link to a homemade HF tx where that very same transformer number is use as a modulation transformer. So, no idea how it works in your audio amp. Out of curiosity, measure the loudspeaker winding resistance and let me know.

https://www.stationqrp.com/. The first part, an 8W am tx download shows the picture of your transformer. I wonder if it has a low impedance winding for a public address loudspeaker, assuming it was used in a mobile transceiver originally? That would make sense.
Rob
The resistance across the speaker terminals is 0.9 ohm.
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Old 27th Mar 2023, 11:39 am   #15
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Default Re: 6V6 ECC83 push-pull amp project

Fuse is in the secondary HT CT link to 0V/chassis.
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Old 27th Mar 2023, 11:43 am   #16
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Default Re: 6V6 ECC83 push-pull amp project

Fuses were sometimes used to connect the centre-tap of the HT secondary winding to chassis.

Are there two diodes in the HT rectifier?

I think I can see four diodes. Are they in series or do the form a bridge?
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Old 27th Mar 2023, 11:58 am   #17
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Default Re: 6V6 ECC83 push-pull amp project

Quote:
Originally Posted by trobbins View Post
'''
The 10k input stopper and 1M grid leak are typical for a guitar input amp.
...
But the the lack of a cathode resistor doesn't seem typical to me in a guitar amp. There will practically be no bias. For grid current biasing the value of 1M would be very low anyway, but more importantly, bias can hardly develop since most of it would leak away through the input/guitar/stompbox).

An electric guitar can produce quite some output when strumming it hard. Some bias (by using a cathode resistor with an electrolytic capacitor parallel to it) seems advisable to me.
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Old 27th Mar 2023, 12:06 pm   #18
RogerLLL
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Default Re: 6V6 ECC83 push-pull amp project

Quote:
Originally Posted by GrimJosef View Post
The HT supply circuit is slightly different from the one you've drawn.

You show a 40uF capacitor in parallel with the 22k dropper resistor. In fact I suspect that the can holds a dual capacitor - perhaps two 40uF units. The caps' two negative terminals will be connected together inside the can and will then be connected to the can. This, in turn, will connect them to 0V. The two terminals connected by the 22k are the caps' positive terminals. So after the bridge the components along the HT rail are:

33ohm current limiting resistor
First 40uF capacitor to ground (conventionally called the 'reservoir' cap)
270k bleed resistor to ground (to safely discharge the caps when the power is switched off)
Connection to the output transformer centre-tap
22k HT dropper resistor
Second 40uF capacitor to ground (conventionally called the first 'smoothing' capacitor)
Connections to the 6V6 screen grids and V2b anode load resistor
10k HT dropper resistor
8uF capacitor to ground (second smoother)
Connections to all the other ECC83 anode load resistors

Cheers,

GJ
I've made all these mods as you have listed. You're correct in the cap can containing 2x 40uF caps. I did think that just a single 8uF smoothing cap was a little under what it should be!
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Old 27th Mar 2023, 12:08 pm   #19
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Default Re: 6V6 ECC83 push-pull amp project

Quote:
Originally Posted by trobbins View Post
The pot on J1 likely goes to 0V at the end that also goes to the tone network/stack, which is likely a Fender style tone stack variant or similar (see http://www.duncanamps.com/tsc/ and http://www.kldguitar.com/Navigation/...ack-Works.html).

The 10k input stopper and 1M grid leak are typical for a guitar input amp.

No bypass cap is used for a global feedback connection, as that 2k7 needs to see both the output signal being fed back, and the input signal coming to that stage.
You're quite right, I'd missed that ground connection.
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Old 27th Mar 2023, 12:14 pm   #20
RogerLLL
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Default Re: 6V6 ECC83 push-pull amp project

Quote:
Originally Posted by Gabe001 View Post
Some work has already gone into that amp already and I seem to recall that Jim said that it works. I assume you're going to try it 'as is' first once the valves arrive?

In case you have issues with the output transformer, I have a spare effectively new 8k to 4/8 ohms push pull opt suitable for your 6v6 output valves which you can have for cost of postage.

You will find the increase in volume quite remarkable. For a guitar amp you may want to cut down on the feedback and see it you prefer it that way (or use an on/off feedback switch).

Good luck

Gabriel
Thanks for the offer Gabriel! I may well take you up on that. I will definitely try as-is once the 12AX7s arrive and see how it sounds. One thing I'm not sure about is what impedance speaker needs to be used, as there's no data for the OT. Not sure how critical this is in terms of potential damage to the OT if the wrong impedance is used? Thinking just go for an 8 ohm for testing.
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