Issue With Bassman 5F6A Build

[Guitarnut]

The turret where your red-blue bias tap is wired seems much too close to the turret with the second bias cap negative. There must be absolutely no connection between them. I think you were better off with the original eyelet board...

I just ordered some replacement caps and I have diodes and 15K here so I'll just rebuild the whole bias supply. I may add some eyelets to get some distance between the two.

Peace,
Mark

[Guitarnut]

I removed the bias AC from the turret and temporarily soldered it directly to the diode. Now I have -67.9 at the anode, and -53.1 at the second cap and 15K as well as the junction of the 220K resistors. And -51.7 on pin 5 of V4 and V5.

Just a short causing all the weirdness.

I'm gonna count that one as a step forward.

Peace,
Mark

[martin manning]

So those two turrets were actually touching? Maybe you can find a way to move the turret for the cathode end of the diode over to the edge of the board. Drilling a new hole without removing the board would be easy enough; the trick will be swaging it in place. The bias voltage may need some tweaking, but it's for sure pretty close.

[Guitarnut]

So those two turrets were actually touching? Maybe you can find a way to move the turret for the cathode end of the diode over to the edge of the board. Drilling a new hole without removing the board would be easy enough; the trick will be swaging it in place. The bias voltage may need some tweaking, but it's for sure pretty close.

They're not made to touch. Probably more sloppy soldering or a strand of wire sticking out.

I'm thinking about replacing the 56K with a pot to give a variable/fixed setup.

[img:800:600]http://www.crenshawweb.com/bassman/pot.jpg[/img]

Thanks again for all your help! It's much appreciated.

Peace,
Mark

[Guitarnut]

Instead of messing with turrets and eyelets, I just placed the diode inline with the supply AC and shrink wrapped it with the anode soldered to a short run of 18awg and then soldered to the turret where the anode used to be. Everything meters fine at this point. I think I'll still rebuild the bias supply to clean it up some and replace the singed cap.

I installed the power tubes and powered it up. The DBT glows a little brighter but no where near full intensity. The power tubes warmed up and looked good.

Tomorrow I'll get the speakers wired up and add the remaining tubes.

Actually took a few steps forward today.

Peace,
Mark

[martin manning]

Did you have a load connected to the OT secondary when you powered it up with the power tubes in? With the PI tube out there can be no drive signal, but there is always some noise and the possibility of an oscillation.

[Guitarnut]

Did you have a load connected to the OT secondary when you powered it up with the power tubes in? With the PI tube out there can be no drive signal, but there is always some noise and the possibility of an oscillation.

No load attached.

So, plugging in just the power tubes doesn't really verify anything? I need all tubes in to get a proper check on power tube voltages?

Peace,
Mark

[Guitarnut]

I don't get Mojo sometimes; that is about the worse power transformer design than one could spec for a Tweed Bassman, that is the perfect PT to make a Tweed Bassman very loud and stiff.

TM

Hi TM,

What is it about this transformer that you don't like?

What trans would be a better choice? I know there are very expensive options but I need to keep it in the $150 range if I upgrade.

Peace,
Mark

[Reeltarded]

No load? That will end this chase really quick... with fire and tears.

[Guitarnut]

No load? That will end this chase really quick... with fire and tears.

Don't want that...

[martin manning]

Did you have a load connected to the OT secondary when you powered it up with the power tubes in? With the PI tube out there can be no drive signal, but there is always some noise and the possibility of an oscillation.

No load attached.

So, plugging in just the power tubes doesn't really verify anything? I need all tubes in to get a proper check on power tube voltages?

The danger is that without a load to dissipate the energy delivered to the output transformer some very high voltages can develop internally, potentially damaging the insulation on the windings. Shorting jacks on the speaker output are one way of preventing damage, but if you have RCA jacks like the original, they are open-circuit. This is mentioned in the P Ruby start-up procedure. Since you weren't applying any signal to the power tube grids you may be OK, but don't power up with the power tubes installed unless you have a load connected. This goes for all tube amps.

[Guitarnut]

The danger is that without a load to dissipate the energy delivered to the output transformer some very high voltages can develop internally, potentially damaging the insulation on the windings. Shorting jacks on the speaker output are one way of preventing damage, but if you have RCA jacks like the original, they are open-circuit. This is mentioned in the P Ruby start-up procedure. Since you weren't applying any signal to the power tube grids you may be OK, but don't power up with the power tubes installed unless you have a load connected. This goes for all tube amps.

Got it. Thanks!

I did keep a hand on the OT and it never got warm. Hopefully, I'm okay. No fire or tears at this point.

I'll recheck voltages on the OT with just the rectifier installed before going any farther.

Peace,
Mark

[Reeltarded]

It doesn't have to get warm. It can happen in ten seconds or twenty or not at all. Without signal, as Martin said, you are probably alright and NEVER DO THAT AGAIN.

heh

[vibratoking]

NEVER DO THAT AGAIN

Unless you're gonna buy a replacement from Miles Magnetics. I hear they make the best, period correct transformers will all the mojo...bar none.

[ToneMerc]

I don't get Mojo sometimes; that is about the worse power transformer design than one could spec for a Tweed Bassman, that is the perfect PT to make a Tweed Bassman very loud and stiff.

TM

Hi TM,

What is it about this transformer that you don't like?

What trans would be a better choice? I know there are very expensive options but I need to keep it in the $150 range if I upgrade.

Peace,
Mark

Part of the magic with a Tweed 4-10 Bassman is the power supply and the current rating of the power transfomer. Far too much attention is paid to tube rectifier "sag" and not enough attention to the PT that couldn't keep up. The transformer that Mojo sells would be great for a BF or 70's SF, but is far to heavy in the HT rating as listed. I think there are some assumptions that all Fender power transformers are the generally the same and from a vendor like Mojo, it's certainly easier to stock a few generic transformers.

Victoria IIRC at one time sold a proper substitute PT to the public and I believe the Mercury tone clone has the correct HT rating, but is wound for 110/115V primary, unless you get it would for modern mains voltages.

My personal mantra is that I try to include many of the small nuances that contribute to the total original recipe, but you do you, my method may not be for you. If you are happy with your build, thats all that matters. However, I like to make folks aware when I see things that are not really accurate in some way.


TM