801 low power SE amp

[Sheldon]

I just finished putting together and doing initial testing on a stereo 801a SE amp. It's resistor loaded with a parafeed type output. I got the original design from Steve Bench's site: http://members.aol.com/sbench/inp_dri.html

Also linked there are discussions on hum cancelling methods and starved filaments, both of which I used. By fiddling with the components, I was able to get to about 0.5mV on the outputs. I suspect that with an oscilloscope, one could do a little better.

I made significant changes in only two areas. I used a 0.9H choke (cause that's what I had readily available) instead of 4.5H, and so I adjusted the associated cap to 220n from 47n.

The other change was to the value of the bypass cap in the cancellation circuit for the 801 grid. Instead of 2.2u, I found that about 20-40n worked best. I did this by voltage measurent and ear, and could get significantly lower than with the spec.'d cap. I don't know why the value should be so different.

A further note on the schematic I used: I had only one 7v filament supply, so there are two compensation circuits, though not shown that way below, that are both run off the same supply. I know this is not ideal and gives me very limited differentiation for the standard hum bucking pot. It works ok with one well matched set of tubes I have. I will eventually replace the transformer or add an additional 7v supply so that I have completely independent circuits.

Any comments/ suggestion would be most welcome. I am particularly interested in comments on the power supply. It models well for static conditions with about 3mV ripple for B+ and a fraction of a mV for the driver. But I wonder if the impedence of the driver side might be too high. One mitigating factor is that I'm only using these amps to produce the spectrum above about 300hz.

[Sheldon]

Oh, and don't blame Steve for this power supply or the bias supply. These were my concoctions.

Sheldon

[Sheldon]

Bias. I got the transformer idea (common 5V transformer reversed)from Tony on this forum.

[EC8010]

I think you want to correct your diodes in that bias rectifier drawing. If they were really connected that way they'd be short-circuiting the transformer!

[kevinkr]

I am assuming that your rectifier is not really a 6X4 - this is definitely not a good choice at the voltages shown.

Kevin

[Sheldon]

Originally posted by EC8010
I think you want to correct your diodes in that bias rectifier drawing. If they were really connected that way they'd be short-circuiting the transformer!

Oops. I get dizzy after a while using Microcap to draw the circuits.

[Sheldon]

Originally posted by kevinkr
I am assuming that your rectifier is not really a 6X4 - this is definitely not a good choice at the voltages shown.

Kevin

Thanks for the reply. Ok, sow why is that? And I'm not being argumentative. I'll freely admit my ignorance. I looked at the data sheet for the following.

Heater negative with respect to cathode: 450 Volts
Positive ditto: 100 Volts

PIV: 1250V

Current: 210 mA

Transient peak current: 1A


So in looking at those values, my reasoning was that the heater is floating, so I should be ok. The max. swing with the bridge is a little over 600 volts.

PIV is about 650.

Current is only about 25 ma

Transient peak current only exceeds 1A (about 1.2)for a couple of cycles. With PSUD that assumes the diode is hot on turn on, so I would guess with normal warm up that the peak is less than 1A?

So where did I go wrong? Hasn't blown up or acted funny -- yet.

Sheldon

[Sheldon]

Some extra data

[Sheldon]

IMD at around .3-.4w (I think)

[Sheldon]

THD