Finally Finished 2x6550 Head w/Tube FX Loop

[Bob-I]

Excellent build. I'm flabergasted by the work you put into this amp, just the filters looks like a complex build, let alone the rest of that amp. Looks beautiful and I bet it sounds as good.

One question, what bias balance resistors did you use. I put in 68k as I believe 6550's like far smaller values than 6L6's where I typically use 220k. I'm afraid it's limiting my ability to drive these 6550's. What's your take on this?

[David Root]

Thanx for the kind words Skyboltone!

I will post a couple of pics of the other side of the chassis as soon as I can get a memory card for this new camera. Would you believe the internal memory that is built into the camera is only good for TWO pics?! So you HAVE to buy a memory card.

In my book that's sleazy marketing. Panasonic DMC-TZ3. Pity because it takes great pics. I'll get over it.

The wiring on my current build should look a lot better. More or less straight, non-overlapping runs from the board to the tubes, and I had the board made 3/4" wider on the other side so I could run a power line and star ground buss, like Hoffman's boards.

This probably old news to you pro builders, but I have recently been doing full size layouts on paper, having in hand the chassis with all pots, jacks, etc. fitted (not the iron of course), and the turretboard. I draw around the chassis on paper, then mark in the positions of all chassis mounted parts, then draw in the turretboard and all terminals, so I have a layout that looks like an old Fender layout dwg except it's with a turretboard. Then I go thru each tube in turn and draw in the resistors and caps on the board so that the wires from the board to the tubes don't overlap. I start from the preamp end of the board, and no wiring starts until everything checks out OK on the dwg.

This is a 5E7 Bandmaster tweaked for harp, with a second channel from the Gibson GA-40 mic input replacing the normal Fender channel (5879 pentode), and cathode bias for that old time greasy tone. The board is a tight fit in the chassis as the chassis wasn't designed for a modern turretboard, there are potential interference issues with pots & jacks & tube socket pins.

[David Root]

Bob-I, thanx also! You definitely have a feel for what I went through.

To answer your questions, the bias balance resistors are part of the so-called Rich mod double pot post PI MV (elsewhere on this forum). I used 180k resistors in parallel with 250k pots, which comes out to about 105k effective bias balance. I put in .047 coupling caps, so in these respects it's the same as an old Hiwatt. 68k is on the low side so you might well be losing some power compared to 100k or higher.

I have seen 68k bias balance in an interesting hi-fi design with 2x6550, but it did seem to reduce effective output a bit, considering he had 515V on the plates. The designer had 220k in originally but reduced them to 68k in order to improve the DC stability of the amp as the 6550s age.

You might want to check it out, the guy's name is Patrick Turner, an Aussie, at www.turneraudio.com.au and I would highly recommend his site in general as it is largely serious technical stuff with no marketing BS. You are looking for the circuit for his 50-50 integrated amp. He is primarily hi-fi but does know a bit about guitar amps too. I found it very useful.

[Bob-I]

Bob-I, thanx also! You definitely have a feel for what I went through.

To answer your questions, the bias balance resistors are part of the so-called Rich mod double pot post PI MV (elsewhere on this forum). I used 180k resistors in parallel with 250k pots, which comes out to about 105k effective bias balance. I put in .047 coupling caps, so in these respects it's the same as an old Hiwatt. 68k is on the low side so you might well be losing some power compared to 100k or higher.

I have seen 68k bias balance in an interesting hi-fi design with 2x6550, but it did seem to reduce effective output a bit, considering he had 515V on the plates. The designer had 220k in originally but reduced them to 68k in order to improve the DC stability of the amp as the 6550s age.

You might want to check it out, the guy's name is Patrick Turner, an Aussie, at www.turneraudio.com.au and I would highly recommend his site in general as it is largely serious technical stuff with no marketing BS. You are looking for the circuit for his 50-50 integrated amp. He is primarily hi-fi but does know a bit about guitar amps too. I found it very useful.

Great site, thx.

I measured about 42watts output with the 68k, then upped the resistors to 150k and now I'm measuring 81watts, HUGE difference.

[David Root]

That's impressive. Perhaps I should change my 180k parallel to something higher, after all the Hiwatt used EL34s not 6550s.

Let's see, if I changed them to about 470k that would give me about 160k effective bias. I will try that and let you know.

When you changed yours, did it change the tone at all or just bump up the volume?

[jjman]

I'm liking that type of knobs for my upcoming build. Where did you get them? Are they Tele type knobs?

[Bob-I]

That's impressive. Perhaps I should change my 180k parallel to something higher, after all the Hiwatt used EL34s not 6550s.

Let's see, if I changed them to about 470k that would give me about 160k effective bias. I will try that and let you know.

When you changed yours, did it change the tone at all or just bump up the volume?

First off, mine is a bass amp so the comparison is not apples to apples. I feel that the mids are less pronounced and the overall tone is rounder. I'm also clipping a lot earlier, maybe 11:00 with a 200mv sine wave input, so I may move to a 12AT7 PI.

I haven't had a chance to have my bass player try it with the band yet so all bets are off.

[David Root]

jjman, the knobs are nickel plated Tele knobs from Allparts, part no.
MK 0110-001.

Bob-I, that's interesting. I can see the earlier clipping with the higher value resistors. Maybe I'll try something in between 180k and 470k.