Futterman style 6AS7 OTL with other tubes

[Brit01]

I have a local seller with some GENERAL ELECTRIC 6080.


These are going very cheaply.

Worth using for testing purposes? What are GE tubes like?

[leadbelly]

Originally posted by Brit01
I have a local seller with some GENERAL ELECTRIC 6080.

These are going very cheaply.

Worth using for testing purposes? What are GE tubes like?

Make sure there is not a "gotcha", such as they are untested, they look used, etc., and if are truly cheap, BUY THEM NOW. Take a look at eBay for how pricey those tubes have gotten.

[Brit01]

I need to research all the valves he has. He has a huge box of them ranging from 6SL7GT to 6X5's to 6V6's.

Like a treasure chest

[thoriated]

- never going higher than 140 volts B+ and B- rail

I will vouch for this one. I've run my DC coupled OTL for 21 years now at +/-160-165 volts and by far the most common tube failure mode has been 6as7g arcover which has usually blown the internal tube and supply rail fuses for that channel. Not a horrible problem, but I've probably lost an aggregate of more than a dozen tubes (defined as having at least one section with an open internal fuse) to this failure mode compared to three or four of everything else. Also, when one or two 6as7g's have gone, I often don't note the failure right away, but the amp starts sounding progressively anemic as the open 6as7g fuses accumulate, usually asymmetrically on the B+ and B-. So, I've belatedly come to realize that it's a little silly to go for the last iota of power if the cost is declining sound and excess tube replacement over time.

Also, I've noticed that the Soviet tubes are more susceptible to this high voltage failure mode as compared to NOS GE 6AS7G's that I have. 6080's have been notably less robust than either of the above in this amp, so I would run them fairly gently at a moderate supply rail voltage if I used them in an OTL application, such as probably not going into positive grid voltage territory.

[coresta]

You should pre ageing NOS 6080 , especially those built in the late 80' ... I purchased Philips ECG labeled WC : they're the worst i 've ever seen without 24 hours pre heating : lots of dust on the plates , grids are "burning" on a bright white light when operated @ 140volts /110mA on my tester

[Brit01]

I'll take that into into thks.

Probably going to purchase a few GE 6080's this weekend (along with some 20 other tubes! ) from a local seller. Tubes are like gold dust here so have to seize the opportunity.

Otherwise I have to have them shipped from Europe which takes a millenium and costs an arm and leg in shipping.

[whitelabrat]

Going back to the original thread topic, I've considered using a load of 6CG7's as output tubes. I've got a little Futterman OTL headphone amp on my desk that uses 6N6P output tubes, and there's no reason why someone couldn't scale up the design for more output.

The 6080 is rather ideal for the job though...

[Brit01]

Do you have an output cap on your mini futterman headphone with 6N6P's? Do you have a schematic?

I have quite a few 6N6P's and IMHO give a beautiful sweet sound.
I built a little WCF headphone amp the other week with 6N6P's and was quite impressed with myself. (also use them in my Aikido)

[whitelabrat]

I use a great big 100uF film cap to decouple any DC to the output. This wouldn't be practical with a big Futterman power amp driving 8 ohm speakers of course.

Practically speaking you can use ANY audio tube in a Futterman style circuit. More tubes in parallel translates into less resistance as tubetvr mentioned. The 6AS7/6080 is a good choice due to it's already low output resistance. Less tube are needed to drive 8 ohms effectively.

For example I was thinking of the 6CG7 where they would be only good for a couple milliwatts of output, but let's say I had 10 tubes in parallel for one channel. That's 20 triodes in action which would bring down the output resistance, and add up to a couple of watts to the output. I didn't say it was practical, just possible.

You'll need at lot of 6N6P's, but they are cheap, and it is possible. I don't think it will be a trivial task though.

To be fair here's the headphone amp I use:
Bijou

[Brit01]

use a great big 100uF film cap to decouple any DC to the output.

Great. I'll have a look at that later at home.

btw. What film cap do you use that is 100uF?