Bandmaster Reverb and 5U4GB

[sbirkenstock]

Hi everybody,

I just blackfaced a Silverface Bandmaster Reverb 768.
It sounded a little too bold and harsh, so I put a 5U4GB in.
Excellent now!

TubeChart wants a GZ34 though. (1,9A vs 3A)
I measured the heater voltage and it was only 6,15V.
So I put the GZ34 back and the heater voltage was still only 6,20V.
Since the amp was made around 1970, I guess it was more made for 110V?
Today my line was 120.5 Volts.
So I guess it should be rather around 6.5 than 6.2.

Should I worry about that it is not reaching 6,3V?
If GZ34 and 5U4GB both have basically the same result, should I worry about using a 5U4GB?

Next thing:
I removed V5 (Vibrato 12ax7). I thought maybe the 6.2 Volt come up a bit with one tube less.
It came up to 6.22. Not much of a difference.
But: the amp sounded a lot worse now. It got kind of harsh treble and it sounded colder??
Plate voltage for V1 just went from 249V up to 252V removing the V5.
The sound difference is way more drastic than that.
Any explanation for that?

thanks a lot,
Stephan

[xtian]

As you've seen, amp is happier at lower voltages. When you pull out a tube, all voltages go up because there is less demand on the transformer.

It is better to have heater voltages a bit lower than 6.3vAC rather than higher.

Sounds like you're good to go with the 5U4GB.

[sbirkenstock]

Well, I possibly did not express my self correctly in the original question.
I read several times that you can check if a 3 amp 5U4GB is ok in an amp that was designed for a 1.9 amp GZ34
by checking that the heater voltage does not go below 6.3.
If it does, the power transformer might overheat.
In my case, with both rectifier tubes the heater voltage is below 6.3, so I am not sure.

And I agree, 6.1 to 6.3 volt heater sounds better than 6.5 volt or even more.
Several new power transformers have that, and I reduce it with some usually very small ohm but high wattage resistors.

[xtian]

OK, that's much clearer. Most Fender OEM iron is strudy, and should handle the extra current, but you can monitor the temp of the iron and see if it gets hot.

[statorvane]

You may be able to use a 5V4 instead. Similar voltage drop as a 5U4GB, but it only draws 2 volts. There is also the 5R4. Not sure about the voltage drop, but it is also a 2A tube. Pretty sure the voltage drop is between a 5U4GB and a 5AR4.

No new production 5V4s that I am aware of, but there are plenty on E-bay. You can also get NOS from on-line tube retailers for around $30.

[Stevem]

If you did not do such during your black face work then changing out the 250 pf and 120 pf ciramic caps for yellow type Polyproplene types in each preamp will smooth out the high end nicely also!

Are you still using a harsh sounded ciramic cap to feed the PI section?

Your PT should be ok, but if you could get your hands on a thermal gun or get one of those stick on thermometers I would test out have each rectifier in the amp for 35 minutes at idle and then see what temperature changes you find.

[sbirkenstock]

thanks for the feedback
The temperature measure is a great idea.

I was thinking about the 5V4 as well, but can it hold up the voltage?
The Bandmaster is about 470V B+.

The ceramic 250pf in the tonestack: I usually use silver mica in my own builds.
That is smoother, did not try polypropylene yet.
The 120pf is only for the bright cap? Hardly ever use that.
I actually think that 120pf silver mica (instead of 250 pf ceramic) in the tonestack sound pretty good as well.

I figured that Sylvania NOS 5U4GB sound very good. You get them for about 25-30 US$ as Vivatubes.com for a pair.
That is cheaper than new production and sounds better!

[Stevem]

My 54 Gibson GA77 with 6L6s uses a 5V4 but it only outputs 18 clean watts and it's been a while since I looked at the current specs for a 5V4 , but I don't think it can handle the current needs of the Bandmaster no less the voltage!

[tubeswell]

+1 what xtian said.

If the amp voltages were going to sag more than 10% below normal with the 5U4G, that would indicate the PT can't handle the 5U4G. As it is, you've got 6.2V compared to 6.3V, so you should be fine.

[martin manning]

Why look at the 6.3V winding? I’d measure the 5V unloaded and then with 5AR4 and 5U4. If the voltage drop with 5U4 approaches 10% I’d be worried.

[Stevem]

You might concider this, as this provides the sag of a 5u4 with a 5ar4 and should the tube ever blow at a gig just change out the fuse , yank out the bad recto and your back to playing again!