New JTM style build project questions

[casey73]

I'm drawing up a schematic for a JTM45 style amp with a matched quad of 6AQ5 output tubes for a "Mini Marshall" type amp. I've previously built a JTM45 clone that works well. I'm using a smaller chassis than a standard JTM45 chassis. Looking at the traditional 4 jack inputs, I want to reduce it to two inputs. The HI inputs have two 68K screen resistors in parallel that equal 34K, so I could put a 33K screen resistor in for the first 12AX7 input on each channel. I want to include a switch that parallels the High Treble and Normal inputs so I can blend them using the volume controls of each channel like I do on the standard JTM45 circuit. Basically I want to do the cable jump with a switch. Is there anything I'm missing?

[pdf64]

A dedicated switch isn’t necessary https://music-electronics-forum.com/for ... les-almost

[Littlewyan]

Just something to bear in mind is you'll lose clean headroom using 6AQ5 valves. They will be driven into overdrive sooner than KT66s, 5881s or EL34s. And I don't just mean lower volume (You'll only drop by maybe 3-4dB going to smaller valves) but the amp will distort sooner on the volume control compared with the standard circuit. If that's not a concern then you just need to look at what values to use for your PI and what negative feedback resistor to use. I'd go with the Marshall 18W values for the PI.

[casey73]

Thanks pdf64 for your input and schemo. That's a great solution for me.

Littlewyan. I'm designing this amp with lower plate voltage than a JTM45. I'm using 6AQ5/6005 tubes because I have a 20+ NOS stock of them I got for $1 each around 1990. I recently had them tested (https://arizonatubesupply.com/) and that gave me a matched quad and several matched pairs. I'm retired and have built many conventional style amps in the past, a few I own and the rest I sold. Now I like to build non-conventional one-off amps, because I can . The tubes I'm using are GE 6005 tubes made in the USA in 1969. 6AQ5/6005 tubes are a 7 pin version of the 6V6 and have a lower dissipation than the 6V6. I think I'll get around 20 watts out of the quad.

[cdemike]

Should be a cool build! Have you already spec'd out your transformers?

I'm also building a plexi-style amp also with a condensed input section. Not sure if it'll be your cup of tea, but I decided to run mine permanently as parallel triodes with different bias points. One side has the normal 2.7k/680nF "lead" values, and the other has a 1.5K cathode resistor with a network of bypass caps. I'm using the normally-unused sleeve switch on the input Cliff-style jacks to change the cathode bypass cap values between 680nF and 1.5uF to give me my "bright" vs "normal" channel settings. That, and an Orange-style coupling cap switch. I almost never use the normal channel, though, so your tastes may dictate a different approach.

[casey73]

Yes I've picked out the transformers. I'm using Hammonds, 270FX PT, 1760J OPT. Also a 194D choke I got for free when I worked at AES, still in the box. I've got an amp I built 20+ years ago based on the Matchless Spitfire preamp with a parallel 12AX7 triodes for the first gain stage. That feeds a single 12AX7 triode stage driving a singled ended 5881 output. I recently added a variable negative feedback pot. I like it alot.

I was going to use the same preamp circuit for one of the channels in this amp, but decided to keep it simple and am using the plexi 1987 preamp. I haven't mapped out the layout yet but I should have enough room inside the chassis to use all axial filter caps instead of chassis mount can caps. I started using F&T caps years ago because I found the Sprague Atom caps were larger than necessary, I think because they look cool restoring old Fenders. I'm going to use a 5AR4 rectifier to keep plate voltage a bit lower. I've used 6AQ5/6005 tubes in a few other amp builds and found you can exceed the recommended plate voltage a bit as long as you don't exceed plate dissipation.

I bought some Russian made 7 pin ceramic tube sockets with spring wire tube retainers on Ebay. They shipped from Odessa, Ukraine. Took a month to get here.