Blonde Bandmaster

[dgrainger]

I recently acquired a blonde/brownface "Band Master".

Some oddities about this amp:
- Tube chart says "6G7", but lists 6 preamp tubes implying a 6G7-A.
- Circuit board seems to match 6G8-A Twin (56k feedback resistor, 4M7 in vibrato circuit, .047 tone caps, 125P7A PT).
- Bias & rectifier circuit board matches 6G6-B Bassman.
- 45249 Output Transformer like in a Concert.

I've got it playing and it's sounding good, but I have a couple of questions:

1: is it safe to run this at normal wall voltage? Filament voltage is about 6.8VAC at local wall voltage, have to set variac to 115VAC for 6.3VAC on the filaments. Full wall voltage puts B+ about 512V, vs 445V at 115VAC on the variac.
2: is it safe to use this output transformer with an 8 ohm load, since it apparently expects 2ohms?

[martin manning]

1: is it safe to run this at normal wall voltage? Filament voltage is about 6.8VAC at local wall voltage, have to set variac to 115VAC for 6.3VAC on the filaments. Full wall voltage puts B+ about 512V, vs 445V at 115VAC on the variac.

The filament voltage isn't beyond the +/-10% spec you see on tube data sheets, so that's ok. The 512 on the plates is high but manageable if you bias it safely. Adjusting the dropping string might be a good idea to get the preamp tubes down to where they are at 115VAC input. I would increase the 4k7 after the choke to get the PI node back to normal, then the rest should be good.

2: is it safe to use this output transformer with an 8 ohm load, since it apparently expects 2ohms?

I wouldn't keep that unit in there if you want to run an 8-ohm speaker. The primary impedance is 4x what it should be, and you risk arcing somewhere and causing some real damage. That is especially true with the higher than normal plate voltage. Get something appropriate for the speakers you will be running.

[didit]

Hello --

Perhaps stock parallel pair of 20uF 600V primary filter will suffer. Consider replacing with two 80uF >350V in series with necessary balance resistors. An easy mod. F&T make fine 80uF 450V axials that retail for around $10.

Also, are you certain about a Bandmaster or might it just be a Concert that has been repackaged?

Best .. Ian

[dgrainger]

The filament voltage isn't beyond the +/-10% spec you see on tube data sheets, so that's ok. The 512 on the plates is high but manageable if you bias it safely. Adjusting the dropping string might be a good idea to get the preamp tubes down to where they are at 115VAC input. I would increase the 4k7 after the choke to get the PI node back to normal, then the rest should be good.

Thanks! I hadn't considered that I could do that. It makes a lot of sense.

I wouldn't keep that unit in there if you want to run an 8-ohm speaker. The primary impedance is 4x what it should be, and you risk arcing somewhere and causing some real damage. That is especially true with the higher than normal plate voltage. Get something appropriate for the speakers you will be running.

OK, I had initially planned to replace it but had 2nd thoughts when everything seemed to behave OK. Thanks.

[dgrainger]

Hello --

Perhaps stock parallel pair of 20uF 600V primary filter will suffer. Consider replacing with two 80uF >350V in series with necessary balance resistors. An easy mod. F&T make fine 80uF 450V axials that retail for around $10.

Also, are you certain about a Bandmaster or might it just be a Concert that has been repackaged?

Best .. Ian

That would just get me more peak voltage tolerance, right?

Well, the Concert schematic has the .02 tone caps, 8M2 in the vibrato circuit, and a 27k feedback resistor. When I pulled the OT off, there was a footprint of the old original bandmaster sized OT still there. I know this era is bad for non-matching schematics. Most of the solder joints look untouched, so I think it's all factory. But I have no way of knowing.

This had some silver can mallory 40uF caps from the 70s in it, so it's definitely been messed with in the past. 2/3 of the knobs were frozen where someone left it probably in storage (it came out of Florida). Penetrating oil and vise grips got everything but the vibrato channel volume freed, and at least it's easy to find a replacement for that one. Also found a loose ground which may have been the reason it was put up to begin with.

[Stevem]

Why would running that 2 ohm OT at a higher impedance be a cause for potential arcing?

For sure the amp would output a ton less wattage ( like some 60 % !) and that higher load would be far less of a strain on the output tubes, and I mean it would loose buch of punch on the down side of it all!

[didit]

Perhaps stock parallel pair of 20uF 600V primary filter will suffer. Consider replacing with two 80uF >350V in series with necessary balance resistors. An easy mod. F&T make fine 80uF 450V axials that retail for around $10.

That would just get me more peak voltage tolerance, right?

Correct. Just 40uF capacitance tolerant to 900V presuming those suggested F&T. A 525V DC reading almost certainly puts something over 700V in transients on those caps.

Best .. Ian

[pompeiisneaks]

Why would running that 2 ohm OT at a higher impedance be a cause for potential arcing?

For sure the amp would output a ton less wattage ( like some 60 % !) and that higher load would be far less of a strain on the output tubes, and I mean it would loose buch of punch on the down side of it all!

My general understanding is that 1 level of mismatch is fine with tube amps but 2 tends to put things under too much strain as the tubes are operating way out of their expected range of load.

no?

~Phil

[martin manning]

Why would running that 2 ohm OT at a higher impedance be a cause for potential arcing?

For sure the amp would output a ton less wattage ( like some 60 % !) and that higher load would be far less of a strain on the output tubes, and I mean it would loose buch of punch on the down side of it all!

The high primary impedance drives cutoff side peak voltage up, and (forgot to mention) will cause screen current to rise dramatically on the positive signal swing, putting the screens at risk.

1: is it safe to run this at normal wall voltage? Filament voltage is about 6.8VAC at local wall voltage, have to set variac to 115VAC for 6.3VAC on the filaments. Full wall voltage puts B+ about 512V, vs 445V at 115VAC on the variac.

The plate voltage increase at "full voltage" seems very high. What is the line voltage where you are?

[dgrainger]

The plate voltage increase at "full voltage" seems very high. What is the line voltage where you are?

I just rechecked everything to make sure, and I have switched out the OT, now, so I assume that causes things to draw a different amount:

With Variac turned down to have Filament exactly on spec:

• - Line: 113VAC
  - Filament: 6.30VAC
  - B+: 475VDC

Plugged directly into wall:

• - Line: 122VAC
  - Filament: 6.75VAC
  - B+: 507VDC

[martin manning]

That's much better. Check the voltage after the first dropping resistor on full line voltage and see how it compares to the schematic.

[dgrainger]

That's much better. Check the voltage after the first dropping resistor on full line voltage and see how it compares to the schematic.

466VDC, vs 415 on the schematic.

[martin manning]

Looking at the voltages on the schematic, I think you'll have to double that 4k7 to drop another 50V... maybe try 10k? Might need to go to 2W as well.

[Stevem]

466 still seems high!

I bet your output tubes are only idling at 25 ma!

[dgrainger]

I don't have the right resistors in a safe wattage to move forward until i get an order in. Thanks for your help so far guys!