A mod/trix I never seen before

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oj
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A mod/trix I never seen before

Post by oj »

I was looking at Morgan amps and of course I did a search to see if there was some pictures of the inside.
[img:3264:2448]https://ivanrichards.files.wordpress.co ... 35-005.jpg[/img]

Have never seen a cap between two gain stages like on this one.
Any ideas as what is going on ?
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xtian
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Re: A mod/trix I never seen before

Post by xtian »

Don't seen anything odd. Which component are you talking about?
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RB
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trick??

Post by RB »

I think he is referring to the silver mica between what appear to be the plate loads for the first two stages. just before the tone stack
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Post by Stevem »

I do not see what he is talking about, but the issue I do see is that big hot running Cathode resistor up against that filter cap, and what will be below the bypass cap when the chassis is sitting the right way!
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oj
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Re: trick??

Post by oj »

RB wrote:I think he is referring to the silver mica between what appear to be the plate loads for the first two stages. just before the tone stack
Yes!
Between the first plate loads resistor (220K) and the second (100k) there are a silver mica between the gain stages.
I have never seen this done before, so I was wondering if somebody can fill me in on what going on here ?
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Re: A mod/trix I never seen before

Post by dorrisant »

What model of amp?
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oj
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Re: A mod/trix I never seen before

Post by oj »

dorrisant wrote:What model of amp?
I believe is A Morgan RCA35
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roberto
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Re: A mod/trix I never seen before

Post by roberto »

Seems that amp.

Well the PA is clearly a cathode bias. and PI is clear, 100k for each Rp, 1k2 and 47k below, going to 47nF to 220ks.

Before seems like:
220k into 1k5//1uF
22n to Gain
100k into 820R//1uF
TS à la fender with 100k slope and 2x22nF caps.
Volume to 22nF to PI.

The 47pF gives some negative feedback from 2nd to 1st gain stage.

Is it like that?
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Re: A mod/trix I never seen before

Post by oj »

roberto wrote:Seems that amp.

Well the PA is clearly a cathode bias. and PI is clear, 100k for each Rp, 1k2 and 47k below, going to 47nF to 220ks.

Before seems like:
220k into 1k5//1uF
22n to Gain
100k into 820R//1uF
TS à la fender with 100k slope and 2x22nF caps.
Volume to 22nF to PI.

The 47pF gives some negative feedback from 2nd to 1st gain stage.

Is it like that?
yes, and there is also a voltage divider 270K/470K that looks to be after gain, and the EQ is after gain stage 2.

Well I was thinking in terms of negative feedback, but normally we see a big resistor in series with the cap and back to the same stage (like Dumble).
Would this not be a very big signal/gain loss here ? or with a small cap like here only affect the frequency?
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Re: A mod/trix I never seen before

Post by 10thTx »

On the Fender Twin 6G8 amp, you will find a "snubbing cap" at the base of the LTPI plate resistors. It's there to help prevent parasitic oscillations.

It is a 47pf cap and I am thinking the picture you are showing has a 47pf cap there also.

http://el34world.com/charts/Schematics/ ... IN_6G8.pdf

Maybe this is something similar?

With respect, 10thtx
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Re: A mod/trix I never seen before

Post by oj »

10thTx wrote:On the Fender Twin 6G8 amp, you will find a "snubbing cap" at the base of the LTPI plate resistors. It's there to help prevent parasitic oscillations.

It is a 47pf cap and I am thinking the picture you are showing has a 47pf cap there also.

http://el34world.com/charts/Schematics/ ... IN_6G8.pdf

Maybe this is something similar?

With respect, 10thtx
I don't think so, a LTPI works different than two separate gain stages, but if you right then we can get a preamp cut control like on a LTPI :D
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Re: A mod/trix I never seen before

Post by RB »

When I think of feed back I understand that to be from an output to an input. This clearly bridges two outputs. I don't see how that constitutes feedback. It seems like it would create something like mixing/balancing/co-mingling of the two AC components. Would these signals be out of phase? I think not. Inquiring minds want to know.

RB
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Re: A mod/trix I never seen before

Post by oj »

RB wrote:When I think of feed back I understand that to be from an output to an input. This clearly bridges two outputs. I don't see how that constitutes feedback. It seems like it would create something like mixing/balancing/co-mingling of the two AC components. Would these signals be out of phase? I think not. Inquiring minds want to know.

RB
Yes agree with you I see a feedback like you say and with a big resistor in series with the cap.

Still nobody that have the answer ?
Would be really nice if someone that have spice could show what happen with and without the 47p cap.
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Re: A mod/trix I never seen before

Post by martin manning »

It's hardly different from LNFB achieved by connecting the small cap from the second stage anode to the second stage grid. The reason is because there will be a wide-open path from the anode of the first stage through the coupling cap (which is much larger than 47p) to the grid of the second stage. The result is a rolling off of high frequency in the second stage.
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Re: A mod/trix I never seen before

Post by RB »

When I pushed the submit button on my last post I thought I should have added where is martin manning when we need him?

Thanks so much for the clarification.

Randy
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