audiowise,
4-65: to triode strap, or not to triode strap, that is the question.
I built a stereo single ended 45 amplifier to show at VSAC.
(Vacuum tube State of the Art Conference; Friday noon to Sunday afternoon).
I finished the amp at Midnight on Wednesday. I listened to it until 2AM Thursday, mesmerized by the sound, but I had to go to bed.
The next morning, I got up and turned the amp on. One channel Screeched very loudly, the 45 went gassy and glowed purple, I shut the amp off.
Oh no! I did not have any spare 45 tubes.
I had a pair of 4-65 tubes. The 4-65 screen dissipation is 10 Watts, just like a 45 plate dissipation. 4-65 u = 5, and transconductance = 4000 uMho.
Well, I will try to use the screen as the “Plate” of a 45.
Thursday, I started the makeover from 45 to 4-65.
I had to completely re-build the dual DC filament supply (2.5V @ 1.5A to 6.0V @ 3.5A.
I had to use the same 4.8k output transformers, and had to use the same power transformer with the low B+.
The 4-65 screen rating is 600V, and the plate rating is 3000V. My 331V B+ was made for a 45 (275V plate to filament, and 56V filament to grid).
The B+ barely tickled the 4-65.
I tried it with the screen and plate tied together.
I tried it with the plate tied to ground.
There was almost no difference, so I grounded the plate (with the wire and ground lug in plain sight (looked safe, and was safe, grab the plate connection if you want to).
The 4-65 with Thoriated Filament looked Really Cool.
The amp with such low B+ did play, but could not even approach the sound of the 45 amp that
I built (before the 45 went gassy).
The amp was not at all using the 4-65 potential (pun intended too).
I finished re-building the amp late Thursday.
I had to get up the next morning and get to the Friday noon meeting, it was hours away.
If I ever use a 4-65 again, I will do it with different surrounding parts, and a different circuit.
4-65: to triode strap, or not to triode strap, that is the question.
I built a stereo single ended 45 amplifier to show at VSAC.
(Vacuum tube State of the Art Conference; Friday noon to Sunday afternoon).
I finished the amp at Midnight on Wednesday. I listened to it until 2AM Thursday, mesmerized by the sound, but I had to go to bed.
The next morning, I got up and turned the amp on. One channel Screeched very loudly, the 45 went gassy and glowed purple, I shut the amp off.
Oh no! I did not have any spare 45 tubes.
I had a pair of 4-65 tubes. The 4-65 screen dissipation is 10 Watts, just like a 45 plate dissipation. 4-65 u = 5, and transconductance = 4000 uMho.
Well, I will try to use the screen as the “Plate” of a 45.
Thursday, I started the makeover from 45 to 4-65.
I had to completely re-build the dual DC filament supply (2.5V @ 1.5A to 6.0V @ 3.5A.
I had to use the same 4.8k output transformers, and had to use the same power transformer with the low B+.
The 4-65 screen rating is 600V, and the plate rating is 3000V. My 331V B+ was made for a 45 (275V plate to filament, and 56V filament to grid).
The B+ barely tickled the 4-65.
I tried it with the screen and plate tied together.
I tried it with the plate tied to ground.
There was almost no difference, so I grounded the plate (with the wire and ground lug in plain sight (looked safe, and was safe, grab the plate connection if you want to).
The 4-65 with Thoriated Filament looked Really Cool.
The amp with such low B+ did play, but could not even approach the sound of the 45 amp that
I built (before the 45 went gassy).
The amp was not at all using the 4-65 potential (pun intended too).
I finished re-building the amp late Thursday.
I had to get up the next morning and get to the Friday noon meeting, it was hours away.
If I ever use a 4-65 again, I will do it with different surrounding parts, and a different circuit.
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I missed all but the last VSAC unfortunately. I wonder if you and I have crossed paths up here at some point.
I have considered doing a 4-65A single ended pentode with local feedback since the 4-65A seems incredibly plentiful.
I have considered doing a 4-65A single ended pentode with local feedback since the 4-65A seems incredibly plentiful.
I do not know if I met you or not.
I did go to a demo of Gary Pimm's amplifiers in Silverdale in a home (Gary Dahl's?), not at VSAC.
I was at all the VSACs.
At the last VSAC (Vancouver, not Silverdale), I did a presentation of push pull 300B versus parallel single ended 300B.
They both had the same B+, same standing current, same bias, and the effective plate load impedance was the same for each amp (when the push pull output level was set to stay in the class A region). I used a pair of Usher S520 loudspeakers.
The first part of the presentation was using a projector to give the amp particulars.
The second part of the presentation was in a listening room. The Left channel music from a CD player was applied to both amplifiers, with the loudspeakers facing each other, but one out of phase. Then the gain pots were set so that the sound cancelled (meaning the levels were exactly matched.
Then the reversed phase was re-connected correctly, and the speakers faced forward.
Next, the Left channel was alternated, push pull, single ended, push pull, single ended . . .
The idea was to listen for the difference.
Then I was asked by Echo Audio for a pair of amplifiers and speakers (I used my pair of 2A3 se amplifiers, and usher loudspeakers).
Curt of Echo Audio was doing a presentation at the last VSAC. It was to demonstrate the phono cartridge vertical angle setting.
Then I was asked by another group for the same speakers and 2A3 amplifiers for their presentation of compressed versus uncompressed music
listening session, at the same VSAC.
I did go to a demo of Gary Pimm's amplifiers in Silverdale in a home (Gary Dahl's?), not at VSAC.
I was at all the VSACs.
At the last VSAC (Vancouver, not Silverdale), I did a presentation of push pull 300B versus parallel single ended 300B.
They both had the same B+, same standing current, same bias, and the effective plate load impedance was the same for each amp (when the push pull output level was set to stay in the class A region). I used a pair of Usher S520 loudspeakers.
The first part of the presentation was using a projector to give the amp particulars.
The second part of the presentation was in a listening room. The Left channel music from a CD player was applied to both amplifiers, with the loudspeakers facing each other, but one out of phase. Then the gain pots were set so that the sound cancelled (meaning the levels were exactly matched.
Then the reversed phase was re-connected correctly, and the speakers faced forward.
Next, the Left channel was alternated, push pull, single ended, push pull, single ended . . .
The idea was to listen for the difference.
Then I was asked by Echo Audio for a pair of amplifiers and speakers (I used my pair of 2A3 se amplifiers, and usher loudspeakers).
Curt of Echo Audio was doing a presentation at the last VSAC. It was to demonstrate the phono cartridge vertical angle setting.
Then I was asked by another group for the same speakers and 2A3 amplifiers for their presentation of compressed versus uncompressed music
listening session, at the same VSAC.
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i have a pair of those 4-65 tetrodes and a quadie of 4-250 tubes, but i have not decided to do anything with them not as yet...my reasoning is that it will be doubly hard to make an OPT for those tubes as compared to say 6C33c..
i have the 4D32 and the QE08-200 lined up...
right now i have a GK71 single ended pentode to commision....this pandemic has prevented me from doing that...too many people, my kids working from home....
i have 20 pcs of 21HB5 i bought when they were U$1 at Rogalskie's in Florida....planing to make a pp monoblocks with those, haven't decided if triode or pentode mode, although i am more inclined on pentode mode with G2 and G1 supplies regulated...
Your commitment is admirable with regards to winding your own iron.
500VP 300V SG, 100mA into a 5K transformer would get you a hair over 20W. That might make for a lovely little amp.
500VP 300V SG, 100mA into a 5K transformer would get you a hair over 20W. That might make for a lovely little amp.
Tony,
I'm using a compactron input tube (6BN11), taking feedback from a high-impedance output tube plate (826), applying that feedback to input tube cathode, and I'm getting ~20 Ohms driving impedance at the primary of the output transformer. With a cheap 5k:8 Edcor output transformer I'm getting .027% THD at 1W (0.3% @ 19W) and 0.6 Ohms Zout at the speaker terminal.
I'm sure a 4-65 would perform very similarly in a similar circuit and OPT could be an easy one to make.
I did a fairly detailed write-up on my blog if you want more info, which is linked in my signature line.