Bassman Project Problem

[Subjecttochange]

I am working on a Bassman-ish amp with some changes to it. I am in the process of converting it from Tube to SS rectifier... and am running into issues. I have currently replaced the tube with a quartet of 1N5399's and here's an abridged list of my values.

Measurements:

B+: 386V
OT Primary: 373V

Fliter Caps W/tubes
#1/#2 (parallel): 389V
#3: 379V
#4: 343V

Filter Caps W/out Tubes
#1/#2 (parallel): 505V
#3: 502V
#4: 498V

My problem is that I am getting overheating on the plates of the 6L6s... what could be the cause of this (other than the voltages are a bit high)? I got the same problem when I had the GZ34 in it as well... and the only thing that helped was using a Port Arthur Rectifier (G. Weber)... but even that got super hot on the 25W resistors... so something is wrong... just can't quite figure out what.

Any help would be awesome!
Anthony

[jjman]

What is the voltage on the grids of the 6L6 tubes? Is it stable?

[Subjecttochange]

On Pin 4:

V5: 373V
V6: 372V

Though these peak really fast at like 385V then fall sort of slowly to their values.

[Structo]

Pin 5 would be the grids, what is the negative voltage there and does it fluctuate?

[Subjecttochange]

Yeah... I was trying to get Pin 5 before my last post. I used shrink tubing around the joints at Pin 5 since they are directly attached to resistors--> PPIMV. Difficult to get the little alligator clips in- and my straight probe is not working.

The voltage on the Grid is all over the place- but less than 15V.

Whats the plan of attack?

[Structo]

Did you copy a Fender circuit and if so which one?

Most of the Bassmans show around -40-45v at the junction of the 220K bias splitters.

Bassmans can have 425v on the OT center tap.

[Subjecttochange]

I based it on the 5F6A but have the following changes:

Already Done:
-Super 5F4 preamp (in place of a bright channel)
-Ef86 Voxy channel (in place of normal)
-PPIMV (Dual Ganged connected to the two grids to caps)
-No NFB

What I am trying to do:
-convert to a SS rectifier/solve the grid problem
-maybe even change it to cathode biased- since I think it would help out the EF86 channel (it's a bit too clean as of now).


This is the layout I loosely followed for the Poweramp

http://ceriatone.com/images/layoutPic/f ... 100508.jpg

I would say that I suspect the PPIMV- but the problems existed before I installed that.[/u]

[Subjecttochange]

Oh yeah... I used a 650VCT PT instead of a 680VCT as in that particular layout. not that it much matters.

[mlp-mx6]

You gotta know what the bias voltage is on the grids. It should be a negative voltage. This is CRITICAL for setting the tube operating point.

Measure it with no tubes in.

[Subjecttochange]

It looks like... at the junction of the 27K and 15K resistor in the bias circuit... the voltage is between -44V and -57V without tubes in. But at the 220Ks it jumps around too much to say.

[Subjecttochange]

Would it help at all if I just switched to cathode bias? I was thinking about doing that anyway.

[Andy Le Blanc]

going to a SS rectifier youll lose the voltage drop of the tube rectifier,
you'll have to reexamine the bias and grid volt etc...

[Subjecttochange]

Right. Those are the values with the SS rectifier.
The grid was/is still out of whack.

Would you recommend using zeners to drop the voltage a bit? Then use that to work off of?

[guitardude57]

You might have a bad solder joint or miswire somewhere....or your DVM has a bad lead. The voltage should be stable. As is, would be impossible to set bias IMO.

If the joints and wiring are good.....maybe try a large wattage resistor (25 or more watt) 10k-20k between plates and screen on B+ rails. Somewhere there is an unbalance, perhaps pulling too much current and overheating things.

I am running 467 volts plate and 410 volt screens on a TW inspired circuit with 6L6's, and a 300R shared Cathode, with absolutely no probs.

[Subjecttochange]

Oh I forgot to mention... without the "Port Aurthur" Rectifier Tube configuration... the rectifier tube would blow every time.

After I installed the P.A. rectifier solution the amp would function- but the two 25W resistors would get hotter than hell (easily hot enough to cause severe burns!)... but there was no overheating on the 6L6s.

It didn't seem right so I figured going to SS would make things better/easier... as well as give me some extra room.

Ok guitardude- you're saying I should get off of the "lowering the voltage horse"? I thought it was nit-picky. I am going to go and heat up the joints to see if there's something I can do.