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Old 1st February 2006, 04:02 PM   #41
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by Tony
how about the 6ba6/EF93 pentodes? i still have a bunch of them.
These are remote cut-off so I believe would not be suitable...
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Old 26th February 2006, 10:25 PM   #42
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Default 6V6 Variant with Hammond 1608

After finishing "Baby Huey" - final arrangement was 18K for cross coupled "shunt feedback set" resistor with zero global feedback, I moved on to building a 6V6 push pull amp for niece Kristy.

Due to low HT supply I was stuck with fixed bias for the 6V6s. This meant 100K for 6V6 grid to bias supply resistors. This with the lower gm of the 6V6 (compared to EL84) suggested that the Baby Huey circuit would not be suitable.

I therefore tried ultralinear plus cathode feedback for the 6V6s and 6SL7 for the diff amp front end. This required separating the 4 Ohm secondaries of the Hammond 1608 rather than parallel connecting them as per manufacturere instructions. In order to get the anode loads on the 6SL7 diff amp up to a reasonable value (150K) I had to fit direct coupled mosfet source followers on the diff amp output. The finished amp had a -3dB HF rolloff at 23kHz and was just subjectively slow with no real attack.

On the weekend I removed the cathode feedback allowing "correct" connection of the OT secondaries and rewired it to the Baby Huey circuit but retaining the MOSFET Source Followers between the diffamp and the 6V6 Outputs (47K for source follower Source to 0V resistors). What a dramatic improvement. HF -3dB point now >50kHz and the amp is lightening fast and has the same huge bass authority. The shunt feedback divider used was just scalled off the original EL84 schematic in accordance with the diffamp anode resistors change. NO global feedback.

That is:
Diffamp anode loads: 150K
Shunt Feedback divider: 39K from each 6V6 Anode and 13K for the cross connect.

The use of the MOSFET source followers has two advantages:
1) allows use of low Rg1 on the 6V6 which in turn allows use of fixed bias.
2) isolates the 6V6 input (grid + miller) capacitance shifting the high frequency roll off much higher.
AND one disadvantage:
It requires a separte decoupled supply for the source followers.

So if you want to use 6V6 or any other tube in fixed bias for the output all that is really required is to add direct coupled MOSFET source followers between the diffamp and the output tube grids. The 6SL7 was NOT a great choice for the diffamp but I wanted to stick to octal tubes on this amp (for looks only). If you use 12AX7, ECC803S etc. you can use all the original values from the "Baby Huey" posted design.

Cheers,
Ian
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Old 26th February 2006, 10:44 PM   #43
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Hi GingerTube. Can you post a corrected schematic of your final design? What output transformer are you using?

Thanks...
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Old 26th February 2006, 10:45 PM   #44
EC8010 is offline EC8010  United Kingdom
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But the cathode feedback should have reduced the output resistance, allowing a lower impedance load on any given secondary. Are you certain you had negative cathode feedback and not positive? (It's so easy to get it wrong that I normally try it both ways round and measure the gain in each case to ensure that I have the right connection.)
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Old 26th February 2006, 10:51 PM   #45
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Hi gingertube,

I looked at splitting the secondaries of a Hammond for cathode feedback as well, in my case a 1628SE connected to an EL84. I notice Hammond shows the same schematic diagram for the secondaries of the single ended 1628 as the PP 1608. Before wiring it to the cathodes my standard paranoia kicked in and I measured the secondaries to confrm they conform to Hammond's diagrams. In the case of the 1628 they don't at all. There's continuity between all secondary windings. On the odd chance my Fluke DMM was having a bad reaction to the inductance I applied 9 VDC to one winding and measured the others to ground. 9 volts appeared on all.

To be as fair and accurate as possible, in the case of my Hammond OPTs the wiring diagram is for illustrative purposes only and doesn't represent the actual winding schematic. It may explain the abysmal performance above 10 kHz. I recommend double checking yours as well if you haven't already.

BTW, I ended up flipping the polarity of the primary and connecting the ground end of the EL84 cathode bypass directly to the 8 ohm speaker post. So far I like it a lot, cathode feedback doesn't appear to generate the high harmonics I've seen of global. Spice sims also suggest a much smaller bypass cap can be used (15 uf in this case with a 270 ohm cathode resistor) and still retain solid output to 20 Hz. Measurements confirm it.
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Old 26th February 2006, 11:03 PM   #46
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by rdf
... cathode feedback doesn't appear to generate the high harmonics I've seen of global.
That's my experience too. It tends to be even more effective at suppressing higher order harmonics than lower. The downside is that maintaining HF stability becomes harder.
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Old 26th February 2006, 11:21 PM   #47
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Some "off the cuff" comments.

The cathode feedback was connected correctly - confirmed by Zout measurement. The Ultralinear plus cathode feedback did sound OK (or even reasonably good) - it was detailed with good bass, it just did'nt have any real attack which was'nt noticable on all music selections but stood out like the proverbial canine testicles on some other selections.

With the 4 Ohm secondaries of the 1608 separated rather than parallel connected I was jusy not getting the HF response out of it AND there were some nasty resonances at quite low frequencies which were very touchy to supress using zobels from anode to screen connections of the OT. I think that these resonances were also getting back into the amp via the cathode feedback.

By wiring the OT secondaries in parallel as per Hammond instructions and wiring the output stage as Ultralinear plus shunt feedback the observed resonances were at least X2 higher (up around 68 to 70 kHz). I'm beginning to think that cathode feedback requires VERY good quality OTs to work.

I changed one channel first and listened for a while. I thought that the ultralinear plus shunt feedback (the "Baby Huey" arrangement) was just marginally better. Then I did the second channel and changed my mind - it was MUCH better. One of those mods associated with HF response where it impacts speed/attack and imaging where you have to do BOTH channels to appreciate the full improvement.

I'm still going to have a mess around with Ultralinear plus Cathode feedback in the future BUT I will be using VDV2100 CFB/H Toroidal OTs with bandwidth to 250kHz.

For inexpensive OTs then the Ultralinear plus Shunt feedback is "the go".

I'll draw up a schematic for the 6V6 version as time allows and post it. The EL84 version remains as per the posted schematic here.
For your info - the EL84 version is better but then its built with better OTs so ......? The shunt feedback trades output tube gm for reduced rp so its my view that the EL84 version (gm is double the 6V6 gm) will always be a bit better given the same OTs etc.

Cheers,
Ian
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Old 26th February 2006, 11:35 PM   #48
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Quote:
Originally posted by gingertube
I'm beginning to think that cathode feedback requires VERY good quality OTs to work.
Yes, I think so too - you seem to trade distortion for HF stability.
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Old 27th February 2006, 12:03 AM   #49
rdf is offline rdf  Canada
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So in the case of the 1608 the 4 and 8 ohm secondaries really are isolated windings? My interest is this aspect may point to the cause behind the issues with the 1628SE.

Hi EC8010, I've only tried cathode feedback with a James 6123HS/EL84/fixed bias and a Hammond 1628SE/EL84/cathode bias and haven't seen anything so far to suggest instability. The James is flat to ~65 kHz and the Hammond is a mess above 10 kHz but to 100 Mhz both were clean as a whistle save for the normal 120 Hz. What combination of tube/OPT parameters should I be cautious attempting? In the pile of next projects is an AMI/Rowe PP jukebox amp for donor iron. The secondaries include 70 volt windings and it's my intention to try them for cathode feedback.
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Old 27th February 2006, 12:25 AM   #50
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Hello rdf,

I'd love to offer you some guidelines about cathode feedback but I've still not seen anything that even suggests useful predictions. It seems that you just have to try it and keep a close eye on the 10kHz square wave response. The only thing I will say is that CFB seems to work better with high-mu valves than low mu (for the same amount of feedback) and that it's unpredictable with pentodes.
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