Anode to Anode cap coupling?

[Dave the bass]

A question from the daft-looking kid at the back of the classroom etc etc etc.

WRT this schematic -> http://www.advancedaudiorentals.com/doc ... _schem.jpg

Whats the function of that cap coupling the 2 anodes of the ECC83? Its rated 0.00002F if I'm reading it correctly.

[andrew Ivimey]

Phroaaaaah if that cap breaks down. If it doesn't well maybe it just cleans up HF or summary.

Right now await a sensible answer.

The circuit, I like though.

[Nick]

I would guess it's to reduce gain at HF. You can think about it like a cap to ground from each anode but the way it is it will only see the signal voltage as the anodes are at about the same voltage.

[Dave the bass]

Thank you.

I was reading its function wrong then. I thought it was for forcing the 2 anodes to balance seeing as the whole bit of kit is a balanced tone control used in a sound recording.

A work mate is obsessed (his own admission) by this bit of kit and I must admit his audio enthusiasm has rubbed off on me a bit. Lunchtime chats are fun again

[Nick]

Yep, all the eq stuff is on the left hand side between the two transformers, The valve stuff is a balanced line driver, in effect a small push pull power amp. The cap in question will operate like the cap and resistor network across the first stage load resistor in these schematics and act to reduce gain by reducing the load impedance at HF.

[izzy wizzy]

Is that to negate the effect of phase change causing oscillations due to feedback?

[Nick]

Is that to negate the effect of phase change causing oscillations due to feedback?

That would be my assumption. Ref my post about control theory and stability margins. At some point the delay imposed on a signal as it passes through the amp will mean that the negative feedback becomes positive feedback. If at that frequency the gain of the amplifier over the feedback loop is > 1 the amp will oscillate, if its close to 1 it will ring. Adding the cap will mean the gain falls before the positive feedback point is reached and the amp remains stable.

[izzy wizzy]

Just for fun, here's a modern take on that kind of circuit http://www.kandkaudio.com/wp-content/up ... ne-amp.pdf

[Nick]

Just for fun, here's a modern take on that kind of circuit https://www.audioasylum.com/forums/Kand ... /4428.html

I get a 404 for the pdf in that link

[izzy wizzy]

I get a 404 for the pdf in that link

Sorry, should have checked. Edited post above.

[Dave the bass]

Work chum would like to build a copy of the Pultec as originals are mega bucks and highly sought after but the iron alone is pricey of course.

Klark Tecknic have done an affordable passive eq/valve gain and buffer stage and managed to market it for £199.

Its a clever copy.

[Nick]

Just looked, and as expected Sowter do the EQ bits, and I guess they will do the rest of the iron as well. Not that that makes it any cheaper.

https://www.sowter.co.uk/eqinductors.php

Just noticed, the balanced feedback winding on the output TX is nice.

[Dave the bass]

Yeah, ta, I was gassing with him last night about Sowter and sent links to their website, and CineMag too for TXs in the states. I never realised there was such a big movement in the DIY Recording gear world, I didn't know it was even a 'thing'. Talking and working with this fella is fun and informative, he's a really creative individual and rides a Royal Enfield, no really.... hey Bruv! *CYBER KNUX*

A year ago a German DIY enthusiast did a crowd sourcing project to create 30 kits to build a clone. The technical video used to sell the idea makes mention of the feedback winding from the Peerless OP tx. There's a bit of hyperbole but the intentions are good (IMO), 7.43 is the basic breakdown on the cct but the whole vid is worth watching if you've the time and inclination.



and here -> https://analogvibes.com/wp-content/uplo ... gvibes.pdf

[ed]

Yowsa...I've just realised the pultec eqs are available in vst for my daw....I'm installing them as we speak...I'm all fired up as I haven't added anything to the studio setup for ages.....fingers crossed.

[Dave the bass]

Thats the closest chum-Antonio has got to owning one too Ed, he's using a software version (summat like Puig?) in one of his mix/recording set ups.

He sent me a screen shot of his virtual EQ's.