v76m circuit questions

[Potato Cakes]

Hello, everyone,

For years now I have been casually gathering schematics and information regarding a v76m channel strip that I wish to build by next year. One of the things that I have asked about in the past is what would be the affect on the circuit regarding audio if dropping resistors were used instead of the two inductors? I also recently noticed that both of the EF804s's use dropping resistors directly from the B+ supply so what was the reason to use inductors instead of resistors for the E83F and EF804? I realize that this was designed in an era where component sizes were much larger than ones with the same values as today and the tolerances were greater so maybe that had something to do with it. Another question is couldn't the primary winding of the output transformer be used to supply B+ to the E83F as it is done with tube guitar amps and even 1073/1290 style circuits?

I should clarify that I am not trying to build something that is exactly the same as an original v76m as I feel such pursuits are in the end not worth the time or money to gain that extra 0.1-1% "better" sound. The v76m is know for sounding big and awesome. I am using this circuit as a foundation to create something that is also big and awesome. And equally as important, something very quiet.

Thanks!

Paul

 

[rock soderstrom]

One of the things that I have asked about in the past is what would be the affect on the circuit regarding audio if dropping resistors were used instead of the two inductors?

That is an interesting question. I think that the anode chokes have an influence on the sound. They are clearly superior to anode resistors in terms of efficiency. The E83F used (note correct spelling) is not particularly current potent and would have more problems driving the transformer without a choke. Why this rather weak tube (and strong step down transformer) was used remains the secret of the developers. In my opinion, a stronger tube (and topology) has simplified matters and would not have required such a complex 9:1 OPT.
Edit: After further research, I would no longer write it that way. If you take everything into account (time and available space and tubes) and weigh it up, this is a clever but also expensive solution.

I realize that this was designed in an era where component sizes were much larger than ones with the same values as today and the tolerances were greater so maybe that had something to do with it.

I don't think so. I think (german?) tradition played a bigger role. Certain things were just done that way and complicated inductors were more accessible because every larger company had its own department for them.

Another question is couldn't the primary winding of the output transformer be used to supply B+ to the EF84F as it is done with tube guitar amps and even 1073/1290 style circuits?

This is where the size of the SE OPT really plays a role. DC in the primary winding requires an air gap and forces a large core diameter for a broadband signal. This means that large transformers are needed for good bass reproduction. The V76's parafeed topology has an advantage here, the capacitor blocks the DC, the transformer has fewer problems with core saturation and can therefore be built much smaller.

I should clarify that I am not trying to build something that is exactly the same as an original v76m as I feel such pursuits are in the end not worth the time or money to gain that extra 0.1-1% "better" sound. The v76m is know for sounding big and awesome. I am using this circuit as a foundation to create something that is also big and awesome. And equally as important, something very quiet.

Yes, that's an interesting question as to what you can really leave out without compromising the sound and performance of the amp too much.

Good luck and keep us up to date.

 

[Potato Cakes]

The E83F used (note correct spelling)

My brain said EF83F my hands said EF84F. Correction made to original post.

The V76's parafeed topology has an advantage here, the capacitor blocks the DC, the transformer has fewer problems with core saturation and can therefore be built much smaller.

Good to know. This will save me a couple of days of troubleshooting.

I'm still a ways out from starting acquire parts. I'll report back.

Thanks!

Paul

 

[rock soderstrom]

My brain said EF83F my hands said EF84F. Correction made to original post.

There is still one F too much. E83F . This F, for pentode, after the numbers is part of the European tube code, it means that this tube is a special quality tube. It should be noted that the EF83, otherwise the normal quality version, is in this case a completely different tube and not compatible! This is a rare exception to the rule, don't ask me why...

 

[Potato Cakes]

HA! Well, I like to have extra F's on hand in case someone runs out.

Sometimes my eyes see one thing but my brain reads it as something else.

Second correction attempted.

Thanks!

Paul

 

[beatnik]

The choke fed output stage is typical of these circuits. I would stick to the original design.

Lundahl LL1667 / 15mA seems like a good candidate for the output choke and readily available.
Someone recently posted a V76 build with these chokes.

 

[ruffrecords]

The choke fed output stage is typical of these circuits. I would stick to the original design.

The choke provides 6dB of extra headroom because it means the anode could swing as high as twice the HT voltage.

Cheers

Ian