Re: One for the bottleheads

How have you used the 26? I would think it was a good candidate for
fillament bias, a 15R resistor and a 16v DC supply should be somewhere
near.>>

I'm using balanced topology, since I have a balanced out of my DAC
digital board (Chris Found V-DAC4). The DAC goes straight into the
valve stage. I'm using a Morgan Jones type cascode CCS under each diff
pair. I've baised it for 100v on the anodes with a HT of 180v, 27k
anode resistors. I first ran each pair of 26 fils off a SLA 6v 12Ah
battery with resistors to drop the voltage, but since the 26s are just
over 1 amp the draw of 2 amps at 1.5v meant the batteries lasted about
3-4 hours, and in any case the voltage was progressively dropping. That
was just to get them going. I then connected the fils in series, but
found that my collection of 26s drew different filament currents so it
was hard to get the voltages to match, on top of the fact that I also
had to balance the anode voltages. I found two pairs that were in the
ball park, then connected one pair to a Thurlby Thandar bench supply
(PL154). This proved to be much quieter than I thought it would be
after reading horror stories of SET users with high efficiency horns
struggling with hum from their 26 line stages. I bought another s/h
PL154 online and am just waiting for it to arrive - it's useful for
filaments since it's 0-15v at 4A. I've bought some 12a and 71a off ebay
and I can use the bench supplies to try out different filamant voltages
and arrangements. Sounds very detailed. I think next step is a
permanent DC supply for the filaments. I've been downloading the
LT1083/4/5 applications *** as one possibility. Then there are the
Ronan Regs which have good reports. If the bench supply can produce
virtually inaudible hum, then that saves going down the battery road. I
think there's another and maybe different problem with some of these
DHTs, which is microphonics on the battery valves like 30, DC90, 1LE3.
These have very low filamant draw (like 50mA at 1.4v) and the filaments
are reputed to 'sing' audibly. The 26 should be free of that particular
issue, though not exempt from the more common causes of microphonics. I
found one amongst mine that was probably loose inside somewhere, since
it vibrated audibly when you put it up to your ear and gave it a tap
with your fingernail. The rest seem OK.
Reports from users of the 26 in a line stage vary from very satisfied
(better than 27 or 76, even 10y in one case) to 'not quite the sound I
want'. Gary Pimm moved on to other valves, others preferred the 27
driving the 26, and one user preferred the 30. Definitely a 'contender'
one could say. At some stage I should try it as the input tube in my
amp, where I have 2C22/7193s at present in SRPP. The other contenders
for the amp are the 12a, 71a and 46 which I should be getting some time
soon. I put a bit of money into these DHTs seeing the rate they are
going up in price, figuring I wouldn't lose on them. 71a was 7 dollars
- now it's closer to 25 dollars. Radio hams still need them, so there's
a small but permanent market. It's a drag having to buy everything in
the USA but there we are. The European DHTs are rarer still. They can
be bought, but I'm very suspicious about availability of stocks, and a
pal who's tried out 1LE3a gave them up because of extraneous noise
problems. There are a few UK users of these DHTs I know of, but it's
fingers on one hand.
Let me know how things go. When my other DHTs arrive and I can try
them, I'll tell you the results! Andy

.