Question Regarding a Partial Feedback 807 SE

[JPeitzman]

I am looking to build a partial feedback 807 SE amplifier based roughly on the RH807. I would run the 807 at 350v (54mA) with 250v (2.5mA) on the screen. Now I have been looking at the front end and thought there could be some improvement to be had. There are two routes I have been considering. The first is a pentode front end using either a 6AU6A or 6J7. The second would be a cascode front end using something such as a 6BQ7A. I have very little experience with cascodes and, although being similar to pentodes, I don't know what advantages if any it would hold over a 6AU6A.

I am drawing up a schematic as I type so I can post at least the 807 back end to get some feedback. The other question I had for now is pretty newbie, but when I figure the size of the screen resistor (the one with the ?) I figure in some current for the zener string correct? Therefore it would be figured for roughly 100v drop and 2.5mA screen current plus whatever current for the string right? Thanks in advance everyone!

[hemgjord]

I don't have any experience with cascodes but they have a poor PSRR.
The screen dropping resistor needs to be calculated so tha the zener string doesn't drop out of regulation at full power. According to data sheet for the 807 screen current goes up to 15mA at full signal in A1. This means that you have to calculate with 15mA + a few to keep the zeners happy

/O

[JPeitzman]

As for PSRR; I am working on the power supply but so far am planing on having a CLC section after the rectifier for the B+ for both channels. After that I am planing on a pair of simple shunt regulators for each channel for the C+ to isolate the input sections from the outputs. Hopefully independent shunts for the inputs would help with any PSRR problems. It is (PSRR that is) a concern though, something I must consider if I use a cascode.

I hadn't accounted for the current draw at full signal, I guess I hadn't considered it for SE, silly me. I am wondering now if a simple 10M45S current source loading the zener string instead of the resistor would offer any improvement. It wouldn't cost all the much, all of about $2.00. I have also thought about a screen regulator similar to what SY uses on his Red Light District amplifier (attached). I suppose the best way to find out whats best would be to just try some different arrangements and see what works and sounds best. However from what I understand (which isn't all that much) pentodes benefit in almost all cases from a clean stable screen supply. Thanks for the help!

[Miles Prower]

Originally Posted by JPeitzman

There are two routes I have been considering. The first is a pentode front end using either a 6AU6A or 6J7. The second would be a cascode front end using something such as a 6BQ7A. I have very little experience with cascodes and, although being similar to pentodes, I don't know what advantages if any it would hold over a 6AU6A.

I don't think you can go wrong either way. The 6AU6 isn't glamorous, but it is a reliable workhorse that does just about everything well: CCS, RF amp, audio amp. It's mainly a question of finding a decent loadline, then tweaking for performance. They also make really good pseudotriodes.

As for cascodes, I already used 6BQ7s for that, but as a cascoded LTP. It sounded really good. However, watch out for the 6BQ7s that have the series connected heaters: these ring like bells. The Motorola and Sylvania 6BQ7s with parallel connected heaters don't have that microphonic problem.

I am drawing up a schematic as I type so I can post at least the 807 back end to get some feedback. The other question I had for now is pretty newbie, but when I figure the size of the screen resistor (the one with the ?) I figure in some current for the zener string correct? Therefore it would be figured for roughly 100v drop and 2.5mA screen current plus whatever current for the string right? Thanks in advance everyone!

You might need screen stoppers for the 807s. These might require it for stability and freedom from snivets.

[Yvesm]

Originally Posted by hemgjord

. . .
The screen dropping resistor needs to be calculated so tha the zener string doesn't drop out of regulation at full power. According to data sheet for the 807 screen current goes up to 15mA at full signal in A1. This means that you have to calculate with 15mA + a few to keep the zeners happy

Agreed, but just 15mA. No more !
This will protect the screen in case of overload.
Add a cap (10µF) accros the zeners to supply some short peaks.

Yves.

[hemgjord]

Good point Yves. I usually have screen stopper that seem to provide protection as well.

/O

[JPeitzman]

Thanks everyone! I'm thinking for now that I will run with the 6AU6A, but I'm sure I'll give the 6J7 and/or the cascode a shot too. I will get a schematic up soon for comments; I'm a student so some weeks are more slow going than others.

[Shoog]

The RH807 is highly compromised as a design - so you are doing the right thing by substituting a pentode. I would be looking to pass something like 6-10mA through the 6AU6.
The 100K partial feedback resistor in the original RH807 seems a bit marginal to me, I would want to lower its value to increase the feedback. If you run the 6AU6 at the same plate voltage as the 807, you can vary the feedback resistor and leave everything else the same and see what sounds best. If you run the 6AU6 at about 200V you will have to account for the fact that the anode current will be partially supplied through the feedback resistor - and compensate the plate load accordingly dependent on the feedback resistor.

Shoog