Looking for 18W schematic using 6973's

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JMPGuitars
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Re: Looking for 18W schematic using 6973's

Post by JMPGuitars »

Why did you run two separate sets of leads for the heaters? That's normally done with one chain to keep the leads shorter, and reduce potential heater noise.
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Re: Looking for 18W schematic using 6973's

Post by BillHarder »

This will be a point to point build and the pre-amp heater wires will be buried under all the resistors and capacitors making rewiring nearly impossible in the future. Therefore, I wanted them independent of the output tube heater string so if/when I make changes to the output tube wiring, it has no impact on the preamp heater wires. At least until I am done experimenting with output tubes.

If I find it is causing hum, it is pretty simple to go back and attach the pre-amp string to the output tube string, shield them, twist them, etc.

I reversed the twist in the two heater strings to try and optimize noise cancelling where they run in parallel back to PT. At least that is my theory. Will see if it works or not.
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Re: Looking for 18W schematic using 6973's

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I'm late to this topic. but wondering how the project is coming along. I have built a Supro 24 Lite into a Hammond AO-39 chassis, and then rewired V1 for a 5879 pentode so now its kind of a Gibson GA-40/Supro 24 mashup. A very cool sounding amp. I just finished an 18W TMB and after I play it awhile I intend to rewire the power section for 6973/6CZ5/6CM6. A jumper across pins 1 and 3 allows you to use all three of those tubes, and the 6CM6 gives you the 6V6 tone without needing socket adapters. I'm not a real fan of EL-84 amps.
Last edited by Brianmac on Sat 02/01/20 7:47 am, edited 1 time in total.
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Re: Looking for 18W schematic using 6973's

Post by JMPGuitars »

Brianmac wrote:
Fri 01/31/20 4:31 pm
I'm not a real fan of EL-84 amps.
Most EL84 amp circuits aren't very good. None of them IMO are anything as good as these 18 watter circuits. Do a lot of playing with that amp, and maybe record something with it before you change it. Play with it cranked if you can. The tubes don't matter as much as the circuit.
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Re: Looking for 18W schematic using 6973's

Post by Brianmac »

JMP - I'll take that advice and try to be patient. I've been tweaking the preamp cathode resistor and bypass cap values a bit, and upping grid stopper to the PI as it seems to me that there is too much preamp gain in the TMB channel (extra gain stage) for the power tubes to shine.
For other EL-84 circuits I have built a Spitfire, a C ghia, and a simple AC-15 - so far I do like the 18W better. The Spitfire in particular does not get much character until my ears are bleeding. Some like that I know, but I like my amps to have some grind when used without a pedal.
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Re: Looking for 18W schematic using 6973's

Post by JMPGuitars »

Brianmac wrote:
Sat 02/01/20 7:43 am
JMP - I'll take that advice and try to be patient. I've been tweaking the preamp cathode resistor and bypass cap values a bit, and upping grid stopper to the PI as it seems to me that there is too much preamp gain in the TMB channel (extra gain stage) for the power tubes to shine.
For other EL-84 circuits I have built a Spitfire, a C ghia, and a simple AC-15 - so far I do like the 18W better. The Spitfire in particular does not get much character until my ears are bleeding. Some like that I know, but I like my amps to have some grind when used without a pedal.
It sounds like you're on the right track. Have you purchased an attenuator yet? That might be your favorite toy if you get a good one. It really allows the amps to sing without destroying your hearing.

Thanks,
Josh
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Re: Looking for 18W schematic using 6973's

Post by BillHarder »

Here is where I am now. Still waiting on a few parts and pieces. Getting close though. Thanks to all for the suggestions and discussion. I am optimistic this will be a cool sounding amp once it is finished and tweeked.

I have made a few changes based on suggestions and neccessity. My original 2500R power resistor had a lead fall off. I have replaced it with a 3000R mallory I had on hand so the plate voltages everyone was concerned about will be a little lower. I lowered the value of the second power resistor from 22k to 20K using 2 10K's to keep the remaining downstream voltages pretty close. Will adjust as neccessary once I get it powered up and run a voltage check.
I deleted the double home run on the filament string as it was starting to get crowded and that was an easy solution to make space.

Still need to mount output jack, bias adjust pot and rectifiers for bias as well as wire up the input jack and channel selector switch. Need a few plates to blank off holes etc. On the home stretch now.
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Re: Looking for 18W schematic using 6973's

Post by zaphod_phil »

crgfrench wrote:
Thu 01/02/20 7:13 pm
I have some EL84 to 6973 adapters I use in the Fullerwell.
I had a quick look at the 6973 specs, and it appears these tubes have less gain than EL84s, much more like an octal power tube. You will therefore need the PI to give them more push. 470R/20k is recommended in the PI tail instead of the 820R/47k of a regular 18W. 6973s are beam tetrodes and will probably give a 6V6ish vibe. I reckon the amp will probably sound pretty good!
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Re: Looking for 18W schematic using 6973's

Post by zaphod_phil »

Brianmac wrote:
Sat 02/01/20 7:43 am
\I have built a Spitfire, a C ghia, and a simple AC-15 - so far I do like the 18W better. The Spitfire in particular does not get much character until my ears are bleeding. Some like that I know, but I like my amps to have some grind when used without a pedal.
The Spitfire's defficiency mainly appears to come from its Matchless/Vox style PI, with the 1.2k cathode resistor. That both kills a lot of PI gain, and also cold-biases the PI, so that it breaks up with a fizzy character, due to its hard clipping. The Marshall 18W has an 820R cathode resistor which seems to drive the power tubes much better,and without the fizz. I think 680R might also work well, or even better, in the PI for some great warm overdrive. FYI, Ken Fischer used 750R cathode resistors (two 1.5ks in parallel) in his EL84-based amps.

The Spitfire also has a 0.0022uF coupling cap after its first preamp stage which contributes to a very trebly sound which some folks might consider "ice-picky".
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Re: Looking for 18W schematic using 6973's

Post by zaphod_phil »

JMPGuitars wrote:
Thu 01/02/20 6:44 pm
6V6 and 6973 are interchangeable using an adapter...
6973s, like 6BW6s, are basically just 6V6 tubes in a 9-pin format. Therefore, to use 6973s in a Marshall 18W circuit, you need to modify the PI tail to 470R/20k to give more push to the power tubes, instead of the 820R/47k of a regular 18W with EL84 power tubes. The Marshall 18W's 8k OT will also work fine, as with regular 6V6s. And don't forget to change the power tube pin connections, to suit the pin-out difference between EL84s and 6973s.
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