Filament Q&A

[dhuebert]

Hi it's me, I'm back. It's -20C outside and time to put the Triumph away and think about nice warm tube amps. I was looking over the schematics and have come up with a couple of questions.

You suggest using the resistors in the filament circuit to give a ground, even with the Hammond, which has a center tap. Talk to me, what is the difference?

In the schematic you have a line that says "spare ground", is this intended for the filament ground/center tap?

I have read and heard that a 10% drop in filament voltage can increase tube lifespan by up to 300%. What are your thoughts?

Can you see any advantages to a DC regulated filament supply? Thinking about it, it seems to me that you would lose the filament ground, I guess I need to know the purpose of the center tap/resistor split ground to answer this question.

I keep seeing these huge resistors in the power supply. What is the purpose of this beast? If it's to drop B+, why not use a more appropriate transformer? If it's to control current spikes into the caps, would a choke be OK? Is there something I haven't thought of?

I went to a local music store and bought a set of 6BQ5's for $8 CDN each(6$US). Three were Sovtek and one was MESA, are you cringing? Will guitar amp tubes make Diana Krall sound like Jimi Hendrix?

[Shannon Parks]

Hi it's me, I'm back. It's -20C outside and time to put the Triumph away and think about nice warm tube amps. I was looking over the schematics and have come up with a couple of questions.

It's 30C outside here - forced to visit mom and dad in the USVI. Forgive me if I seem incoherent as I am loaded up on Nitrogen and Presidente.

You suggest using the resistors in the filament circuit to give a ground, even with the Hammond, which has a center tap. Talk to me, what is the difference?

We could skip a ground reference all together and let the filaments float, or even tie one side to ground (like the Eico HF-86), but center taps do lower hum - at least common wisdom and tube bibles do say so. But center taps on transformers are never perfect as far as being truly 'centered'. A milliohm meter might be needed to decide this, but I figured a pair of 1% resistors would be a fool proof voltage divider. This probably makes no difference for the Hammond folks, but is handy for the Dynaco folks (no CT). The original ST-35 did reference the filaments to the EL84 cathode voltage, btw.

So the answer to your original question: I suggested Hammond users to go ahead and use the 100 ohm resistors so they could experiment between the CT and pseudo-CT if they so desired (don't use both at the same time though!). Unless someone has extremely efficient speakers, though, this is probably never meaningful. But part of the DIY philosophy.

In the schematic you have a line that says "spare ground", is this intended for the filament ground/center tap?

Correct.

I have read and heard that a 10% drop in filament voltage can increase tube lifespan by up to 300%. What are your thoughts?

I believe Steve Bench has an excellent article on his website regarding lowering filament voltages. Google it. Everything is relative, as power output, THD, etc., is affected. I think if is worth experimenting with. Otherwise, I'm happy with -/+5% variation. My test ST35 runs right at 6.3VAC (original PA774), and I put in some voltage drop resistors (ones in the manual) and it runs at 5.9-6.0VAC.

Can you see any advantages to a DC regulated filament supply? Thinking about it, it seems to me that you would lose the filament ground, I guess I need to know the purpose of the center tap/resistor split ground to answer this question.

Not in this amp. You would use a bridge rectifier on the 6.3VAC taps, huge cap, big power resistor (for the proper voltage drop) then feed the filaments to gorund. The Hammond could do it, not the Dyna.

I keep seeing these huge resistors in the power supply. What is the purpose of this beast? If it's to drop B+, why not use a more appropriate transformer? If it's to control current spikes into the caps, would a choke be OK? Is there something I haven't thought of?

The 10W is way over rated to help those that are 'improvising'. Using a choke is great; the Dyna chokes are cheap (see Ned) and knock the ripple down.

I went to a local music store and bought a set of 6BQ5's for $8 CDN each(6$US). Three were Sovtek and one was MESA, are you cringing? Will guitar amp tubes make Diana Krall sound like Jimi Hendrix?

Some dislike the Sovteks. The distortions measurements emulate my RCA 7189 tubes - but at 12W instead of 14W. I like them. :)

Shannon

[dhuebert]

Thanx for the above.

I pulled the power amp out of my console stereo-HiFi, if you recall, 6BQ5 single ended. I spent Sunday morning making mods to it and tube rolling. I have three sets of 6BQ5 and three 12ax7's. One set of 6BQ5 had brown stains on the inside of the glass, Gar Gillies assured me these tubes were no good, onother set were the original from the amplifier and the last set are the new Sovteks. I went through all the tubes and found the supposedly blown ones to be the best. I was right about the sovteks, they made Diana Krall sound like Jimi Hendrix.
All through the testing the amp had very poor control over the bass, somewhat flabby sounding. It occured to me that the power supply was running the whole system and maybe didn't have enough juice to control the low frequency stuff. I disconnected the power to the rest of the console (radio and preamp) and the bass tightened right up. The problem now is that the original tubes make a crackling sound on loud passages. The Sovteks still sound like guitar tubes, and the blown tubes are definitely the best. They have a buttery smoothness to them that really pleases. BTW the new Sovtek 12ax7 was the best sounding of the bunch.
After about ten minutes of listening, however, the sound quality really started to drop off, I think the 40 year old power supply caps are shot. I'm going to start replacing caps and resistors and doing more sound tests in the weeks to come. I especially want to get rid of the ceramic caps in the sound path. A fun way to spend a couple of hours!