Im considering changes to my PA 26-66

[IggyT]

Greetings to all! its been a while! combination of Retiring, moving and other life stuff plus a pandemic all took my time. Anyway..
I built a variant of the PA 26 about 8 years back and was pretty pleased with it at the time. I drilled the chassis..built the head etc and liked the sound. it didnt get a lot of play because it was just too loud before it really sounded sweet to me and also the noise floor was a bit annoying. So I have a few thoughts and questions... I am attaching the schematic from Scott Rubel I believe (which used EL 34s) and also the schematic from Stratman which used KT 66 tubes.
1) The original layout on a very small chassis put the PT (Hammond 273BX) closer to the OT (Hammond 1760J) than I would have liked so I thought I would rehouse the amp in a larger chassis to better distance them
2) while doing that I also wondered how the amp would respond if I tried a tube rectifier rather than the diode rec I did use.
3) I hard wired V1 without a switch and was wondering how I might get a bit more gain at a lower volume
4) last question: I used a 1000uF cathode cap as per Stratmans diagram but noticed that Scott specified a 100uF in that position. not sure if that was a typo and that the correct value should have been 1000uF
thanks for any suggestions
Iggy

 

[Lowerleftcoast]

1) You can re-house it. I like to keep amps small or original. You can use the headphone test to hear if the OT is picking up noise from the PT. I would check it on the larger chassis as well. It is kind of an eye opener to do this test. Sometimes transformers can be fairly close when they are oriented well.
2) The tube rectifier will probably knock off some voltage and might result in some sag. Be mindful of how many amperes the rectifier will use of heater current.
3) You are getting about as much gain as it will give at this point. 220k and higher plate resistors combined with higher cathode resistors will give somewhat of a raspier sound which some amplifiers use (Carvin for one). That sound may/may not be to your liking. You may be able to save some of the signal strength by using a 1M tone pot.
4) Power tube cathode cap values are kind of *to taste*. I would think 100uF is too much. Start at 10uF. It is easy to parallel caps together to get higher values, The values add when paralleled. 10uF with 22uF in parallel = 32uF, so you can alligator clip in values to find what you like.

 

[Lowerleftcoast]

I mis-read the schematic. It appears 1/2 of V1 is not used. You can tap into the other 1/2 for more gain. Lower volume would require a master volume or an attenuator.

 

[IggyT]

I mis-read the schematic. It appears 1/2 of V1 is not used. You can tap into the other 1/2 for more gain. Lower volume would require a master volume or an attenuator.

thanks Lowerleftcoast; currently both halves are being used in parallel so I could feed the second half from the first.

 

[IggyT]

1) You can re-house it. I like to keep amps small or original. You can use the headphone test to hear if the OT is picking up noise from the PT. I would check it on the larger chassis as well. It is kind of an eye opener to do this test. Sometimes transformers can be fairly close when they are oriented well.
2) The tube rectifier will probably knock off some voltage and might result in some sag. Be mindful of how many amperes the rectifier will use of heater current.
3) You are getting about as much gain as it will give at this point. 220k and higher plate resistors combined with higher cathode resistors will give somewhat of a raspier sound which some amplifiers use (Carvin for one). That sound may/may not be to your liking. You may be able to save some of the signal strength by using a 1M tone pot.
4) Power tube cathode cap values are kind of *to taste*. I would think 100uF is too much. Start at 10uF. It is easy to parallel caps together to get higher values, The values add when paralleled. 10uF with 22uF in parallel = 32uF, so you can alligator clip in values to find what you like.

thanks ; regarding point #4 I am speaking about the 1000uF cap from pin 8 in parallel to the 330 ohm resistor: One diagram specs the 1000uF 25V cap I used while the original schematic shows a 100 uF 25V cap in that position. I was wondering about the impact of that difference.
iggy

 

[Lowerleftcoast]

That is the cap I was referring to as well. I don't think you will hear any difference between 100uF and 1000uF.

Check out the formula Blencowe provided in the article below. Scroll down to "The cathode bypass capacitor".

The Valve Wizard -Small Signal Pentode

It shows 100uF and 1000uF pass all audible frequencies. 10uF would cut off some sub bass.

 

[IggyT]

G

That is the cap I was referring to as well. I don't think you will hear any difference between 100uF and 1000uF.

Check out the formula Blencowe provided in the article below. Scroll down to "The cathode bypass capacitor".

The Valve Wizard -Small Signal Pentode

It shows 100uF and 1000uF pass all audible frequencies. 10uF would cut off some sub bass.

Got it; thanks again!
iggy

 

[printer2]

thanks Lowerleftcoast; currently both halves are being used in parallel so I could feed the second half from the first.

That is what I would do, maybe not all the gain in the world, take a page out of Marshall's amps, have one cold biased.

 

[IggyT]

so here's my progress update
I partially dismantled the old amp and moved it into a new larger chassis with a socket for a rectifier tube. Here are my results: ALL readings through the bulb limiter.
1) NO tubes at this point. line voltage is ~123 V AC and all connections seem fine (fuse and switch all show 123v) ; 120 v pilot light lights up.
2) I installed a "hot" Sovtek 5Y3 to see what the voltages might be. I read 4.4 v AC across rec filament and 5.7 v AC on other tube filament pins. Voltage on pins 4 and 6 seemed low at 290vAC.
3) I flipped the standby switch and saw the pilot light flicker a bit. The bulb limiter glowed faintly and became dim after each step. Now I read 380v DC on all B+ locations including the OT pins on the Power tube sockets and also pin 1 on both v1 and v2 sockets. The B+ seems a bit lower than expected with the Sovtek 5Y3 and the flicker on the pilot light caused me to pause for any input people may have on these readings before I try the pre amp tubes.
Iggy