Helmholtz wrote: ↑Mon Apr 14, 2025 8:15 pm
"Gain" channel has 4 gain stages (last stage is a non-inverting cathode follower), so both channels are out-of-phase and mix in-phase at the PI.
A classic example of using the LTPI as channel mixer is the Vox AC30.
Oh, interesting. Maybe I'm confused about which direction my 180 degree switch is on the aby pedal.
I'll have to try my simple aby pedal and compare.
Meanwhile, I used an acrylic paint pen to color in the grooves I made in the leather faceplate. I also tooled a leather name badge. It came out much more Haloweeny looking than I intended. The Hot Cat being a Mark Sampson design, I wanted to use the Matchless font which turns out to be Times New Roman, haha
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I guess I need to cover some of the obsolete back panel info too
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Helmholtz wrote: ↑Mon Apr 14, 2025 8:15 pm
"Gain" channel has 4 gain stages (last stage is a non-inverting cathode follower), so both channels are out-of-phase and mix in-phase at the PI.
A classic example of using the LTPI as channel mixer is the Vox AC30.
I think something is wrong with my Radial ABY. It makes some bad noises at random times. My simple homemade ABY pedal works fine and it doesn't sound out of phase with it.
I tried this hot cat at a rehearsal on stage using my simple ABY pedal. I had to yank one of my pedals that was feeding the clean channel. I thought the amp had a high pitch whine that was right on the edge of almost too quiet to worry about. It was actually converting from my NU-X Dual Delay pedal. It's a dual delay (2 parallel delays in one pedal) that usually runs stereo but I was running one side into the other for mono. In retrospect, I probably should have just tried one side to see if it would be quiet.
Anyways, it scared me that their amp was too noisy... But it was just a cheap pedal that creates a bit of noise running into itself in series:
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Colossal wrote: ↑Sat Apr 19, 2025 9:36 pm
Have you considered trying the Cross-Coupled Garter Bias circuit with this amp?
Yes, that is the plan. If I've done my calculations correctly at full voltage my (only 2 10watts I own) 470r resisters are currently set to 92% dissipation in regular split-cathode bias. I think the garter method cools things a bit so when I order a set of 4 resisters I think I want them to be a bit lower value.
Sorta Hot Cat Full Voltages.png
I think what I'll do is try to experiment with some parallel resisters in this method to get bias in the high 90s to get a value that can switch to the garter method.
I do currently have lots of 470r resisters that are only 5watters. I could also just try them in garter configuration to see how things change dissipation-wise. I assume they would be fine for shorter amounts of time at idle.
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bcmatt wrote: ↑Sun Apr 20, 2025 12:22 pm
I do currently have lots of 470r resisters that are only 5watters. I could also just try them in garter configuration to see how things change dissipation-wise. I assume they would be fine for shorter amounts of time at idle.
At full power you are showing 28VDC per cathode, which is:
28V / 470R = 59.5mA
and power is:
(0.0595A)^2 * 470R = 1.67W per resistor
5W should be fine for the experimental work.
I'm interested to see what the Garter Bias does to your B+.
So, I swapped in a 27K for the 22k in the dropping resister string for B+5 and it brought it tighter to the schematic voltages.
Actually, I noticed all the voltages were a bit closer to the schematic today... I'm thinking because wall voltage is only 120.9VAC today. I think around here we get up to like 125VAC pretty often.
So, before trying the Garter circuit I took measurements. I was getting about 407VDC on the EL34 plates, and about 26V and 27V on the Cathodes. So according to the Tube Bias Calculator, that's only about 83% dissipation with these 470r Cathode Resistors.
When I activated the Garter Circuit, it seemed to push the Plate Voltage up a few volts to 411VDC but the cathode voltage was pulled pretty tight together at 26V and 26.1V. So that is a dissipation of 80.4% if I understand correctly. So, it does cool things ever so slightly. It's hard to know if the difference in plate voltage is really from the Garter circuit or just fluctuations in wall voltage since it is close enough that it could be either.
I think I'll have to use the switch on the back panel to implement a Garter/Split Cathode Bias choice for at least right now while I experiment with dialling in a good cathode resister value. If it is good for regular cathode bias, it should be close enough (though slightly cooler) for the Garter circuit value.
Still experimenting. 400 ohm cathode resisters per tube gives plates at 406V and Cathode voltage of 26 and 27... so about 93%
With 360 ohms cathode resistors, it seems to give 404V Plates and 25V and 26V cathode voltages. That seems to put it in the range of 100% to 104%. I would normally want to play it a little cooler, but I'm thinking the Garter circuit should drop it slightly... also, really, what are my choices for ordering 10W resistor values around that area? I could order 5W 750 ohm resistors, but I'd be trying to cram 8 of them in there for the garter circuit...
bcmatt wrote: ↑Tue Apr 22, 2025 12:55 am
Still experimenting. 400 ohm cathode resisters per tube gives plates at 406V and Cathode voltage of 26 and 27... so about 93%
With 360 ohms cathode resistors, it seems to give 404V Plates and 25V and 26V cathode voltages. That seems to put it in the range of 100% to 104%. I would normally want to play it a little cooler, but I'm thinking the Garter circuit should drop it slightly... also, really, what are my choices for ordering 10W resistor values around that area? I could order 5W 750 ohm resistors, but I'd be trying to cram 8 of them in there for the garter circuit...
Excellent! Thanks for the testing. I think 93-104% should be fine. Yeah, the real estate sometimes gets hard to manage. I am finalizing a design for a similar amp. I need a 30-ish watt clean amp for pedals. I have Vox iron. Did some testing on the PT and I should end up around 370VDC (GZ34 rectified) with the mains running 122.5VAC. I am thinking of trying the cross-coupled garter bias circuit. Been running the numbers.
Colossal wrote: ↑Tue Apr 22, 2025 1:21 am
Excellent! Thanks for the testing. I think 93-104% should be fine. Yeah, the real estate sometimes gets hard to manage. I am finalizing a design for a similar amp. I need a 30-ish watt clean amp for pedals. I have Vox iron. Did some testing on the PT and I should end up around 370VDC (GZ34 rectified) with the mains running 122.5VAC. I am thinking of trying the cross-coupled garter bias circuit. Been running the numbers.
Are you thinking of using EL34s as well?
I am really liking this cathode biased EL34 idea. It's a real pleasure to play so far... In this circuit anyways.
Because tubes are so expensive, I am now getting more attached to this garter circuit idea. I could see myself eventually installing it in most of my amps.
I've ordered some 360 ohm 10watters, but they won't arrive till Friday at the earliest.
bcmatt wrote: ↑Tue Apr 22, 2025 3:22 am
Are you thinking of using EL34s as well?
I am really liking this cathode biased EL34 idea. It's a real pleasure to play so far... In this circuit anyways.
Because tubes are so expensive, I am now getting more attached to this garter circuit idea. I could see myself eventually installing it in most of my amps.
I've ordered some 360 ohm 10watters, but they won't arrive till Friday at the earliest.
Yes, definitely EL34s. I will parallel the input stage, like the Songwriter. Do you find that the single 1n5 coupling cap is enough and balance with regards to low end? I am debating adding either a bass control like the Songwriter, or maybe a 3- or 6-pos switch to change coupling cap values. I am usually a set-it-and-forget-it player so, once I find where I'm happy, I rarely touch the controls. Still some flexibility is nice. Also will had a post-phase inverter master volume but wired as a variable resistor to feed the garter bias circuit. Cut control for Treble.
This garter circuit is interesting.
Dave, what do you mean by wiring the MV as a variable resistor? Why not use the standard PPIMV?
I'm assuming the circuit below (from Broske TCJ blog post) is what you are talking about.
Matt, what is the garter circuit in the EL34 amp discussed in this thread?
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Colossal wrote: ↑Tue Apr 22, 2025 10:55 am
Yes, definitely EL34s. I will parallel the input stage, like the Songwriter. Do you find that the single 1n5 coupling cap is enough and balance with regards to low end? I am debating adding either a bass control like the Songwriter, or maybe a 3- or 6-pos switch to change coupling cap values. I am usually a set-it-and-forget-it player so, once I find where I'm happy, I rarely touch the controls. Still some flexibility is nice. Also will had a post-phase inverter master volume but wired as a variable resistor to feed the garter bias circuit. Cut control for Treble.
Once I got the proper value of 1n5 in that spot, I've found I've been pretty happy with it in my amp. I had been thinking the same thing about adding some sort of control (I also like the Songwriter's Bass and cut combination), but I found that with both cabs I've used it with, I found the bass well balanced now. I've liked this amp with m-65 Creambacks, and have used it with both an open 2x12 and a Mesa Roadster 4x12 (which combines a closed back and open-backed half). Of course, your miles may vary, but you may find you will either are just happy with 1n5, or you could tweak it slightly to your taste and not need to have control. I like that this amp on the clean channel is capable of slightly more gain than the songwriter and I wonder if it's that lack of bass control contributing to a slightly more lively signal. This one having no NFB would suggest that's maybe the reason, but the songwriter's NFB is so small that it's maybe indistinguishable. Of course, I'm comparing my more bastardized versions of these two amps, so ... grain of salt and all that.
That said, I do think the Bass/Cut combo of the songwriter is a beautiful idea. I converted my 5e3 into a 6G3 bright channel with no trem (but keeping the cathode bias). I found it to be such a nice upgrade. I am planning to try the Songwriter Bass/Cut combo instead of the tone control to hopefully give it a bit more control without losing the potential gain on tap. I need to make it a new faceplate anyways.
