audio-talk

Got this breadboarded up at the moment: It is heavily based on Alex Kitic's "RH2A3 with a twist" amplifier - the 6L6 version http://rh-amps.blogspot.com/2014/02/rh2 ... twist.html

It uses simple resistor bias for the output stage rather than an LM317 based CCS and uses VR regulated screens instead of the Zener dropper, otherwise it is identical to Kitic's circuit.

It's good to see you doing something that you so obviously enjoy Steve.

Whats the 100R in the anode for?

Nick wrote: ↑Sun Feb 05, 2023 4:58 pm Whats the 100R in the anode for?

It's there to prevent the wire to the top cap inducing parasitic oscillation. The STC application notes for the 807 recommend it. The resistor is fitted right next to the 807 ceramic top cap connector. I enclosed it with 2 layers of shrink down sleeving to avoid any nasty surprises. Wouldn't be necessary of course, with a normal 6L6 family tube.

pre65 wrote: ↑Sun Feb 05, 2023 4:55 pm It's good to see you doing something that you so obviously enjoy Steve.

Yes I'm enjoying working with these circuits again.

Cressy Snr wrote: ↑Sun Feb 05, 2023 5:06 pm

Nick wrote: ↑Sun Feb 05, 2023 4:58 pm Whats the 100R in the anode for?

It's there to prevent the wire to the top cap inducing parasitic oscillation. The STC application notes for the 807 recommend it. The resistor is fitted right next to the 807 ceramic top cap connector. I enclosed it with 2 layers of shrink down sleeving to avoid any nasty surprises. Wouldn't be necessary of course, with a normal 6L6 family tube.

Would that data advice not have assumed its use @RF output?

I’m just following the book Paul.
I also followed this advice on DIYAudio: https://www.diyaudio.com/archive/blogs/ ... e-807.html under the “other considerations” paragraph.

Here is the relevant section of the STC application note, which gives the anode stopper recommendation and value.

Data is for triode operation make any difference?

The article u linked. Id try “ The latter can be ten spaced turns with an ID= 7/16 inch to provide about 1.0uH of inductance. This is low enough to be of no consequence at even the highest audio frequencies. The coil can be "de-Q'd" by paralleling it with a 100R/2W C-comp resistor (or four 470R/0.5W C-comps in parallel. Mount the resistor(s) inside the coil. The plate stopper needs to be mounted right at the top cap connector, as is the case with any stopper resistors or coils.” As the coil bypasses the resistor dc its win win.

The coil diamater changes the inductance and if you wind the coil from start position with the added turns layered vertically you get better coil frequency response (by reduce capacitance) and greater inductance for the same turn as the diameter increases, so might have to drop a turn, or at least test with L tester, assuming they mean air cored which is probably a given or use foil layered same way turns shouldnt need fettling then.

Sounds like fun.

Liked what they said about dh 1624 too.

But both valves pitifully low powered for audio, again the link shows 6v6 same power ish, in which case for dh theres the 1619.

Neither of which need anode stopper anode comes out the octal base.

Shall I get my coat though?

It was interesting for me though to discover the scenario and thought the combined coil resistor down the middle seemed optimal as holds dc resistance to nothing significant.

Alex could possibly have barked up the wrong tree though with the 807. Back then when he wrote about that design 807 may have been only readily available cheaply valve he could afford. It suited his budget to make a blog which reaped a harvest of attention.

Interesting stuff Paul,
My particular SEP puts out 5.6WPC.

I think the 100R anode stopper is a general recommendation rather than being specific to triode operation. That's how I read the application note anyway.

The 1625 with the 7 pin base and 12.6V heater is something I'd like to incorporate into a future build. I have four UX7 sockets, just need the valves.

Does the Rfb back to the ECC81 reduce the output impedance, then ?
I haven't looked at these circuits for a while.

Cressy Snr wrote: ↑Mon Feb 06, 2023 6:26 pm Interesting stuff Paul,
My particular SEP puts out 5.6WPC.

I think the 100R anode stopper is a general recommendation rather than being specific to triode operation. That's how I read the application note anyway.

The 1625 with the 7 pin base and 12.6V heater is something I'd like to incorporate into a future build. I have four UX7 sockets, just need the valves.

If I find any 1624’s they’re yours. There are certainly some but at the lockup where I had a level collapse on the ground level, so everything is a heap of rubble. Be patient I know theyre in the rubble, protected in boxes very few valves damaged by the fall. Its a retirement plan to sort it. Not time to retire now. But over a few weeks I might find a pait.

Im keeping 1619’s for the first person perpendicular who is a hoarder.

IslandPink wrote: ↑Mon Feb 06, 2023 7:12 pm Does the Rfb back to the ECC81 reduce the output impedance, then ?
I haven't looked at these circuits for a while.

Its mysterious for sure as is Steves mind.

But some pentode amps sound good like the quad 2 and the diynaco. Both of which have highly respected output transformers, quad 2 for its feedback winding. Dynaco I doubt has that but its a very respected quality build.

Well do the math to direct couple the cf to the 807 with a level shifter i.e. absorb voltage in series diodes bypassed with a tiny cap if required for noise reasons. To go a2 aswell.

For the cf use EL84 recommendation of Nick’s. Its a very unusual tube makes it ideal as a cf driver. Ive found EL82’s almost as much transconductance 10 instead of 12 but much cheaper. Im planning using them in my 801a driver one day to benefit from a2.

The higher the transconductance the more suited to driving grid current direct. Lower output impedance you see. El84 pretty perfect. On paper far better than famous circuit used 5687 in ongaku for example. Better sounding valve than valves similar to it used elsewhere like E88CC but would be “move over” in presence of EL84 and similar numbered valves with high transconductzance. Ongaku should be bettered by it. Everything else left the same.