6as7/6080 Se Otl

[revintage]

Seems like these are going to be a pair of sturdy monoblocks !

 

[poynton]

The original design.

Apologies. You did say in a previous post.

Andy

 

[desperateaudio]

Seems like these are going to be a pair of sturdy monoblocks !

When final construction starts it will be a stereo block as i do not have pairs of the transformers unfortunately

 

[Klimon]

Couldn't the shunt feedback circuit you posted be modified to a classical cathode-cathode feedback design (+ volume pot) with satisfactory results?

@Lars: I'ld much appreciate your opinion on this

Cheers - Simon

 

[revintage]

Sorry Simon, didn´t see your question.

It is not possible as the phase is opposite at the grid. If we connect feedback to the cathode it will be PFB. Because of this I drew the second one.

You can see it as a CF with DC-coupling to a grounded grid amp. So still just one amplifying stage inside the feedback loop. Can´t think of a simpler way to do it, sorry.

 

[Klimon]

Thanks. Just asking because of the ammendability of such a circuit would be just within my range of skills; a few sections less/channel would have suited my 6080 stock better. Anyhow, since the tubes probably need to be matched tightly only selection from a large stock or buying matched would be options. I might give your circuit a go later on; the current and heat involved are borderline terrifying

Thx again -- Simon

 

[Fuling]

Fuling, how would the simplest possible PSU for a monblock SE OTL with 5x6080, B+ 135-140V at 1,25A look?

I would definitely use a C-multiplier based on IRFP460 or similar.
Nice slow upstart and with 15-20V to drop the output would be very clean. Let me know if you need parts, I have some stuff that might be interesting for you.

 

[revintage]

Did a sim with a this simple setup:

Transformer 110VAC, rectifier bridge, CRC (4700u 16ohm 10000u) and got 0,7mV ripple at the output.

About -80dB, maybe the CCS helps it up?

 

[Fuling]

I would definitely use a C-multiplier or a choke instead of a 16R resistor. IIRS the PSU in my SE OTL is a CLC filter followed by an C-multi.

 

[desperateaudio]

hope you guys like

 

[desperateaudio]

here's a second

 

[revintage]

Nicelooking, classic breadboard! Shouldn´t be to hard to try in mono with all 6080s in parallell. To draw more current through the tubes the two 40ohms resistors have two be changed. What are the 6080 voltages now: Ua, Uc and in the junction of the two 40ohms?

You could actually adopt the 6SN7 for the grounded grid configuration but with lower senstivity. My guess is in the ballpark 2Vrms for full output. Though if a double-triode should be used, my candidate is 6N1P.

 

[desperateaudio]

I have 109vdc on the plates of the 6AS7's and 490mA per side (3 tubes in parallel per side). The driver tube (6SN7) is receiving the 210vdc to the 100k and 22k plate resistors as designed. The 6AS7's are a little low voltage wise (109vdc as opposed to 135vdc) but I doubt that is causing the distortion and volume control problem posted earlier. I am sure it is the input sensitivity causing that issue. Overall current drawing on the whole amp is 980mA.

Comments are welcome before I pull this apart and build the next one which I reposted here.

 

[desperateaudio]

One other thing, if I am remembering this correctly the vdc at the top of the first 40 ohm resistor was 39vdc. Never took a measurement at the middle will do that tonight.

 

[desperateaudio]

Just did a quick calc. and it must be 39vdc at the top 40 ohm resistor as that is the only way to be at 490mA per side and I know that's correct. So again comments welcome or any suggestions before I disassemble and build the other.

 

[revintage]

You only have 70V and 82mA=5,6W through each triode. They should be biased at least to 12W.

 

[desperateaudio]

I here that. I believe I need to change the cathode resistors what would you suggest? Would love to push the most out of this before moving on. Thx

 

[revintage]

You can´t get more than 109V B+ from your supply?

 

[revintage]

What about this one with a Slagle bifilar 1:1 IT?

With some tweaking one might even get away with only one PSU, feeding the driver from the 6080 B+.

 

[desperateaudio]

You can´t get more than 109V B+ from your supply?

Sure I can tweak a few resistors and bam 135vdc @ the 6as7's. I will report back shortly with that tweak and the associated revised readings at the cathode resistors and current draw.