New Build Idea: 6G4 Based, Cathode Biased

[Chuck H]

To answer on MWJB's behalf...

Is this because of higher voltages than the typical tweed super or other old design with cathode bias 6L6s?

Yup. You'll want to get a few resistors in standard values if you really want to get finite with the adjustment. 250R, 270R, 330R, 390R and 430R. I don't think you'll over dissapate the tubes even with a 250R but you may get better performance with a cooler bias. Listening is the best way to set guitar amp bias. Well, that and looking to make sure the power tubes aren't glowing the screens or plates.

Also what about the issue of the voltage difference between the plates and screens on the output tubes? I think they should be at least 25v apart.

Many guitar amps have the screens very close to the plate voltage. It's not a problem. If we wanted to get the lowest hamonic distortion figure we could pay more attention to the data sheets. There is a good technical reason to keep the screens below the plate voltages but I can't remember the specifics. Some guitar amps actually have the screen voltage higher than the plates and are revered for their tone. Since guitar amps are intended to color the sound with extra overtones, harmonics and other distortions it's usually counter productive to take the data sheets too seriously. The posted voltages in the data sheets are just examples, not ideals. Tubes are pretty flexible and can operate fine with parameters not listed on the data sheets. Saavy techs use the data sheets as a reference guide to the tube in question to get an idea of it's operating characteristics and maximum limits. With a 1k resistor between the plates and screens instead of a choke you should have no trouble keeping the screens below the plates by a safe margin.

[MWJB]

Even the later tweed Fenders often need a 330ohm cathode resistor with today's wall AC voltages, you might get away with 250ohm, or you could try a 5U4 rectifier...even a 5Y3 in cathode bias?

+1 on what Chuck says re screens. The difference between the values you see on the older tweeds and the brown tolex amps is not relevant to method of bias, by the late 50's/early 60's they were looking for more & more headroom & running the screens just (a volt or 2) below the plates worked then...and still does today.

[Rocketrob]

LOL...yea, that's why the values for the dropping resistors aren't filled in on the schem. Thanks for clearing this up for me. The bias resistor will be looked at too; I've got a few different values I can use. The schem I posted shows a 5U4GB and 6L6GCs in cathode bias; so with that setup, 250 or 270 should be OK, no? I have a 5AR4 to use if I decide to get a tighter, cleaner sound. Dropping and bias resistors adjusted as needed. Thanks, RD.

[Rocketrob]

Another Rev

I've decided that moving the first filter cap before the standby switch would be a good idea for less stress on the rectifier tube. See attached drawing to check if I have this correct; it seems simple enough. I just added one eyelet and changed the wiring.

The chassis is all together and wired up, so just need to populate the board and install it. Some pics on the way soon. Thanks, RD.

First cap before standby.pdf

[Chuck H]

That's how I prefere to do it too. One thing to look out for is that the unloaded voltage on that first cap will be much higher than the amps working Vp. So you need to make sure the first filter cap is up to the task.

[Rocketrob]

Progress Pics

Here are a few pics of where I'm at with this. The chassis is pretty much ready for the board. This is a small chassis; same size as an Epi Valve Jr., so things are a bit crowded. I'm a little concerned about the lead dress and parasitic oscillations; we'll see..RD.

[Chuck H]

I think you'll be OK. It's tight for sure but I've gotten away with worse. I think you've done pretty well with a small chassis. This will be a very compact amp no doubt.

[Rocketrob]

I think I made a mistake on the layout drawing. The split load plate resistors for V1b are shown with the feed to the plate taken from the junction of the two resistors. This, I think, is incorrect; the coupling cap to the PI should be connected to that junction, and the two ends of the resistors connected one end to B+ and the other end to the plate. I'll have to fix the drawing. Please stop me if I'm wrong. Thanks, RD.

[Chuck H]

Yep. Good catch. It's surprising how easy it can happen. What sucks is not catching soomething like this before the ampp is built and then trying to retrofit without making it look like crap. In this case it wouldn't have been so bad. Just swing one resistor. You'll draw it better but smething like this:

[Rocketrob]

Layout REV3

Chuck,
That's just how I just how I drew it up last night; at least I'm on the same page with ya. See attached file for Layout REV3; includes last couple of changes. You will see that some values are still blank; I won't fill them in until I know they work well enough.

Board is comming along just a few more things to add. RD.

[MWJB]

Pardon me, I haven't followed every post in this thread but i have a couple of Q's...

What is the 10K resistor goung to the LH tab of the volume pot, treble slope is usualy 100K.

I'd start with 15K for the NFB resistor.

[Chuck H]

Indeed the earlier "schematic" shows that resistor as 100k. So I'll guess that the 10k there in the layout was a typo. Good catch.

[Rocketrob]

Doh!-Good catch indeed;thanks MWJB.
Looking at the schem for a 6G5, with its single 8 ohm speaker, Fender uses a 22k for the NFB. That is what I was planning to use to start. The 15k would give a little more feedback, right? Can't remember which way that goes. Also, it's not shown. but I added a switch to disconnect the NFB near the speaker jack, for some extra raunchyness. Might be good with cathode bias. Thanks, RD.

[Rocketrob]

Ready for first fire.
Question; Since the first filter cap is not switched, should I disconnect the feed to it from pin 8 on the recto to check PT voltages with no tubes in? Can it cause damage if not disconnected? RD.

[Rocketrob]

Fired Up!

Well, got this running at last. Not right out of the box, but pretty close. I had to fix a shielded cable that was shorting. I changed a couple of the dropping resistors to get the voltages more in range. Right now they are: 1K, 470, 51K. I went with a 5AR4 Rectifier, as the numbers looked a little too low with the 5U4GB. Some tweaking to be done for sure, but first impressions are promising. Plenty loud, and a good amount of gain on tap. Split load plate resistors for V1b are 81K-15K; I'll try a 68K-33K combo to see how that sounds. The bright switch is a bit too subtle, may a different cap there. Presense control works pretty well, and the NFB switch is cool; giving a good loose sound, but not into oscillation. There is a bit of hum. I'll try rotating the OT, and/or elevating the heaters a bit. all in all a good begining. Let the tweaking begin!

See atached file for voltage chart. RD.