And that value can be played with. Many of those types of amps I use a 1.2K on the UL connection, just shoot for a screen voltage always at least a couple of volts under the plate has been my experience.

Yes, you just change the bias points, but the output transformers used aren't ideal for KT88 use. I plan to do a series on KT88 mono amps soon!

I would start with the little headphone adapter box if you have never built anything from scratch before. It's cheap, has no high voltage to deal with and will test/hone your basic fabrication and soldering skills.

I need to clarify: when you reduce the size of the cathode resistors to 250 ohms, you HAVE to increase the side of the EL34 bypass caps or you will lose a lot of the low frequency response. Also, this 470uf/35V cap is the min voltage. you can easily sub a 50-100V part. And yes for the driver tube bypass cap you can just move the 220uf cap that was originally on the output tube there. And to the question/complains about not providing a parts list for the A10, it's because there is no change to the parts list. The ONLY difference is rewiring the heaters to use 12AX7 or simply leave the A10 heaters alone and use the tubes it comes with. Attached is a diagram I have shared on the heater wiring several times in other places. Also I have been told there are 2 versions of the A10, one is point to point and the other uses PC boards, they are not the same. These super cheap ones are likely the PC board version which also use different/cheaper output transformers from the point to point wired A10/A12.

C7 is in parallel to an existing capacitor and you really think there is gonna be a measurable voltage drop in an 18ga wire as the plate current varies a couple of ma? GL with your PC board version of this.

As far as "noise levels of the original device", I just finished another one of these, it was built with AC heaters, connected it to a pair of KT120 mono blocks and it's dead silent with my ear to the speaker (RP600M). This was a point to point build, possibly trying to put this tube design on a PC board will end up noisy, especially if it's not laid out correctly. I've seen other PC board pre-amps that are noisy until they were converted to DC heaters. If you build this point to point and it's noisy, it's the layout or the lead dress, not the design.

Just take you time and try to understand what is going on :)

R3 is only there in case the wiper of the pot fails, and prevent the input tube from running away. Many designs don't even include a resistor there There is no reason to increase it to 510k. If there is DC on the input, you have other problems further up the chain! Adding another coupling cap in the signal path as a protection for bad equipment upsteam doesn't seem nessasary to me. And as you said, we already have two out of necessity. C7 is in parallel with the existing 47uf cap on the output of the power supply. R7/C3 isn't going to hurt anything, but the current drawn by the "output" cathode follower is so low, I can't imagine it introducing a signal into the resistance of the 18ga solid copper wire I use. Obviously if we were looking at a beam power tube pulling 60ma, that's a different story.

I'm not a fan of adding another coupling cap in the signal path, what noise are we trying to eliminate? And if someone does this, a 0.68uf is the smallest I would use, probably a 1uf would be better.

Yes, It's the voltage amplifier tube so contributes a lot to the sound.

Unloaded, that's what you will see. Get the tubes drawing current and it will fall into place.

Yeah, took a belated Christmas trip I planned in December but got COVID. Am back in town :)

Aww, sorry I was on a delayed Christmas vacation/trip last week.

This PSU schematic shows the changes with this different transformer. The only thing different between a 274BX and the 374BX is the extra primary wiring and the violet 50vac wire we aren't using.

I need to update that project page, here is the schematic for using the hammond iron. You can use the 266 transformer for the 6.3V as well. If I was doing this today, I would probably use 7V primary for the regulated heaters to give the regulators some headroom, the 266L14. Room was tight retrofitting this into my amp so I used a pair of the smaller 166L6 units, which are 115V only. Hope this helps!

There is no bias circuit, this is a cathode biased amp. To use these hammond transformers, you need two seperate 6.3V transformer windings for the 300B heaters to go to the rectifiers/voltage regulators. You can't run these off the same 6.3 winding, each tube needs their own. "Note where the 50+ volt bias is coming off the 6.3 VAC winding." This is not what is happening. We are applying 50VDC to the center tap (green yellow) of the 6.3V winding. This is elevating the heaters for the 6.3VAC winding to the 6SN7 heaters by applying 50VDC on the center tap, created by a votage divider off the B+ DC power rail using the 357K/47K and the 22uf cap. The green yellow center tap wire is for applying 50VDC not AC coming out. Again there is no bias circuit on this type amp. Ignore the violet 50VAC wire on the 374 transformer, you aren't using it. Red yellow to ground, the other two red go to the rectifier tube. The yellow wires go to reactifier tube heaters, ignore the yellow black.

What driver tube did you used? BTW it looks awesome!

Very cool! I saw a web page about using an Arduino to control a relay attenduator. MAYBE in the future I will take a swing at that. I know how to program those things already which is a bonus.

Try a solen film "fast cap" for the first cap, either a 22 or 33uf should work fine.