Tweed Bassman 5B6 Head Build - Electronics Complete

[ToneMerc]

Powered right up, no smoke


Iron: Magnetic Components Valco 35W

Tubes: Tung Sol RI 5881
RCA 6SL7GT
Haltron 6SC7
RCA 5U4G

As built deviations: shielded cable on inputs, input 1- 1M grid leak, input 2- 5.6M grid leak, 27K grid stoppers at V1, 500 ohm 3W screen resistors, 1.5K grid stoppers on the power tubes.

Voltages: AC Mains 124.1V , 6.84 VAC filament

5U4: 412 VAC, 389.2 VDC, 389.3 VDC, 413 VAC

6l6(V4): 409 VDC, 347.1 VDC, 27.7

6l6(V3): 409 VDC, 346.7 VDC, 27.7

6SL7(V2): 211 VDC, 2.45 VDC, 207.5 VDC, 2.55 VDC

6SC7(V1): 144.3 VDC, 144.3 VDC, 1.95 VDC

TM

[martin manning]

Really nice! Valco OT looks like the real deal.

[M Fowler]

Mike,

Very nice build

[Reeltarded]

Nice!

[Colossal]

Quality!

[M Fowler]

Well since those two guys are chiming in I changed my mind I don't like the copper chassis, how about that

[rp]

Nice build and nice glass there, Pops. I'm bummed as mine won't have a cool hammertone copper chassis Can one even get Hammertone in Italy?

So how about a tone report? Do you think the smaller OT worked out well? I'm hoping you say it's a totally worthless old circuit so I can stop thinking of building one.

[dreric]

Awesome!!!!!!

As usual, a flawless build. Sounds like????

I gotta have one!

Eric

[cbass]

Man that is a nice compact build.

[ToneMerc]

Thanks guys

TM

[ToneMerc]

So how about a tone report? Do you think the smaller OT worked out well? I'm hoping you say it's a totally worthless old circuit so I can stop thinking of building one.

Post Mostem

1. I try to avoid MCI power transformers because they can be all over the place. This particular one is no exception, it's supposely wound at 350VAC at 117V, now with 124V in it's a whopping 403VAC unloaded, which is about 30VAC higher than what it should be for a 124V mains.

For the first few secs until the 5U4 is warmed up it's hitting the first cap with 506VDC and it's a 500V cap. Good thing I used a standby switch afterall. Had I known this was going to be an issue with the PT, I would used two caps in series with equalizing resistors for the first cap node.

2. For this cathode biased circuit, bigger iron is a waste. This is an honest 25W circuit. I think the OT is perfect

3. Rolled several pre amp tubes could not get any 6SC7 anywhere near 160V.When I rolled the good glass stuff, the voltages were in the 135V range. I changed the PI to an early 40's Sylvania the voltage dropped to around 190V there. I changed 6SC7 plate resistor from 249.9K to 180K and that did the trick to bump the preamp up in the 160V range which made a great difference in volume and headroom.

6l6(V4): 405 VDC, 342 VDC, 28.1

6l6(V3): 405 VDC, 342 VDC, 28.1

6SL7(V2): 188.5 VDC, 2.55 VDC, 189.4 VDC, 2.52 VDC

6SC7(V1): 162.8 VDC, 162.8 VDC, 2.25 VDC


4. There's no motor boating, squealing or ghosting, but there is a mild distortion artifact when the tone control is about mid way. I need to put the scope on this circuit.

Tone: I went into this debating about whether to use the stock value caps or change the front end to .05 or .02 and dropping the cathode cap value as well. However, I wanted a stock circuit baseline. All would have been a good, IMHO it is far to bass heavy for guitar.

With single coils i'm not really liking it at all, but it sounds decent with humbuckers running the vol and tone just about wide open. Fully cranked there's some compression which is kind of sweet. I think as it sits it would make a decent harp amp.

5. Should have followed my first mind and had the chassis powder coated instead. I've used the Rustolem hammmer tone before, it takes far too long to cure and you get cold flow around fasteners.

TM

[Reeltarded]

Mild distortion artifact? Is that a complimentary way of describing the sizzle that comes on in a tweed at the end there?

[jelle]

Nice build!!!

[rp]

Post Mostem

Postpartum?

1. I try to avoid MCI power transformers because they can be all over the place. This particular one is no exception, it's supposely wound at 350VAC at 117V, now with 124V in it's a whopping 403VAC unloaded, which is about 30VAC higher than what it should be for a 124V mains.

Haven't used them yet, thanks for the tip and the saved aggravation.

2. For this cathode biased circuit, bigger iron is a waste. This is an honest 25W circuit. I think the OT is perfect

Good, I'll stop thinking about upping the OT in my 5C8.

3. Rolled several pre amp tubes could not get any 6SC7 anywhere near 160V.When I rolled the good glass stuff, the voltages were in the 135V range. I changed 6SC7 plate resistor from 249.9K to 180K and that did the trick to bump the preamp up in the 160V range which made a great difference in volume and headroom.

I had the same experience with V2 (cathode bias modded) in the 5C8 though I tried varying the cathode resitor instead of the plate's. Maybe when Fender measured they had a low measuring plate resistor. I will go try 180K too now that you say it makes a noticeable difference.

Tone: I went into this debating about whether to use the stock value caps or change the front end to .05 or .02 and dropping the cathode cap value as well. However, I wanted a stock circuit baseline. All would have been a good, IMHO it is far to bass heavy for guitar.

Good, if I it build I will go straight with .05 and 25mf. If you make these changes please follow up.

At this point it looks like TAG has stolen The Pedal Steel Forum's and The Hoffman Forum's thunder concerning 5B6 builds

[martin manning]

I try to avoid MCI power transformers because they can be all over the place. This particular one is no exception, it's supposely wound at 350VAC at 117V, now with 124V in it's a whopping 403VAC unloaded, which is about 30VAC higher than what it should be for a 124V mains.

Do you find them consistently high? AFAIK they quote voltages with all windings fully loaded.

For the first few secs until the 5U4 is warmed up it's hitting the first cap with 506 VDC...

I'm trying to make sense of this... I'd have thought the vacuum rectifier would make a soft-start as it warms up?