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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 15 Oct 2019, 16:19 
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Joined: 15 Nov 2013, 10:16
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Location: MB, Canada
Very interested to see how this project progresses as well!


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PostPosted: 16 Oct 2019, 10:15 
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Joined: 11 Nov 2013, 22:35
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Yes very interested as well.


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PostPosted: 09 Feb 2020, 18:38 
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Joined: 06 Apr 2009, 10:08
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Location: US Pacific Northwest
So... I haven't done much with this project of late because I've been preoccupied with a busy work schedule as well as some ground water issues at home which I needed to correct. Now that spring is in the air (at least in this hemisphere) and my ground water issues have been tamed, I thought it might be time to get back to work on this project.

When I left off, I was attempting to determine what I needed to use for output transformers. Normally, based on the operating current and power levels, I would have chosen the Edcor GXSE10-8-5K and moved on. However, I got it into my head that I should check the actual Rp at the chosen operating point and then decide between the GXSE10 and the GXSE15 based on the effective low end rolloff given the primary inductance of the two transformers. Edcor publishes the primary inductance of the GXSE10 at 5H and doesn't publish the primary inductance of the GXSE15. However, based on previous experience I suspect it is somewhat larger then the GXSE10. My thought was to measure the Rp, calculate the low end roll off, and if it didn't meet my liking, go for the GXSE15 instead.

I even developed the test schematic (shown below).
Attachment:
Rp_Test_Circuit.jpg

My thought was that I could fix the bias point, use a signal generator and the UL coupled transformer to induce a change in the plate voltage, and measure the resultant Vp and Ik changes via the output 100k resistor and the current monitor resistor in the cathode line. The dynamic Rp value would simply be the value of Vp over Ik at the UL operating point.

Now I haven't done this test yet and it occurs to me that it may not be necessary. This amp, after all, is going to have almost 20dB of bass boost available via the bass control in the preamp stage. So the question is "Do I really need to care about how flat the response is at neutral tone settings?" In the past, I have found the GXSE10 more than acceptable. For example, in the MarbleWood amp with the 6L6s, I have liked the sound so much that I normally run the amp with 6L6s. And will this bias point be that much different? I also already have four GXSE10-8-5K transformers in my stash. Two additional GXSE15-8-5K transformers will run me about $85 USD plus shipping. Is it really required? So here's my question to the greater tube amp community...

Should I go through the trouble of directly measuring the Rp of the UL stage at the chosen operating point, or simply select the smaller output transformer and let the bass control be my guide to deal with any low frequency adjustment I may want to make?

I am looking for people's opinions.


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PostPosted: 09 Feb 2020, 22:42 
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Joined: 13 Jan 2018, 21:33
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Location: australia
Suncalc wrote:

I am looking for people's opinions.

Matt, assuming low chance of damage and poor sound I would lean toward minimising cost for possibly incremental gain. I suspect the engineer in you would want to make a few measurements to confirm any suspicions though.

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PostPosted: 14 Mar 2020, 08:40 
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Joined: 01 Jun 2013, 09:05
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This is old info,

Still interesting to revise:
http://tubelab.com/articles/component-t ... nsformers/

With anything like 6L6 El34 etc I would go for the higher spec 15 Tx.
But its a gamble on saturation. If you already have some output Tx's you could test them.
I run quite a lot of bass through my equipment on SE and use cathode FB AKA >>PYE.

Its only my opinion but a 10 watt Tx is more like EL84 or 6v6. But even then I use XSE15.
I guess it depends if you are going to run film explosions through it :hot:

Again the output winding spec is a pain where 8 ohm is better than 6 ohm with cathode FB.
It depends what rocks your boat in the old days people always say the equipment output Tx was under spec, but there were some pretty big tx's even in HMV record players using N78 output tubes.

You can probably get away with smaller on PP due to the flux cancel across the Tx, but SE :confused: .

Years ago I used some old paper dielectric transformers I found in an old stereogram on some waste land. I tested them and they were OK.
There is a difference with the dielectric and you can hear it. I notice someone was making old school Tx's with paper insulation but I can't recall who at the moment.

So if its old school I assume you will use CC resistors<<its worth a test I think you might be surprised. And or CF.
Its interesting to note how old school had a "thing" for high meg ohm resistors :D and multi strand flex wire.
Obviously its on low gain circuits.

Regards
M. Gregg

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PostPosted: 14 Mar 2020, 09:26 
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Joined: 01 Jun 2013, 09:05
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Just for interest,

STDST silver multi-strand wire sounds good with CC and CF resistors. :D
CC series signal and CF signal to Gnd.
I went through the" I wanted a system that gave me sound like I remember from the past but with more resolution".
The number of times I have seen someone buy a leak or quad then rip it to bits change everything then sell it because they can't get the sound. LOL.
Its interesting to note the little secrets on some old equipment like capacitors with stripped insulation on the can then clamped to gnd. Where the cap has a grounded case changing the circuit spec. (but its not on the drawing). I visualise the tech smoking a woodbine and saving green shield stamps for the next sale. :D

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M. Gregg

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PostPosted: 14 Mar 2020, 20:24 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, I have grounded the cases on some caps. One kind, Russian K40Y-9s can be soldered too if need be. :)

Good listening
Bruce

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