silvertone reel-to-reel player to series string amp

[thetragichero]

picked up a non-working  silvertone 5238 stereo reel-to-reel player for a song. i've been itching to do a series string amp for awhile (box of 25L6 and an isolation transformer staring at me). my goal is for something in a bluesy/dirty vibe, no nfb just nice and raw. will use the two 8ohm alnico speakers inside the cabinet (if they hold up) to get a 4ohm load and have a 4k reflected primary impedance
cathode follower before tilt tone control as i have read pentode preamps do not like  tone controls that load them down too much. should i go 12au7 (with slightly tweaked phase inverter values?) since i have a box of em pulled from organs? i'm thinking of bypassing the trem cathode with an led that will double as an indicator light (no desire to seek out a 120v lamp plus it's much easier to mount an led bezel). anyway, looking for thoughts/critiques. for the series string dropping resistor i'm assuming i'll need (120-50-25-18)V/0.3A = 90 ohms, power rating 0.3A * 27V = 8.1W, so 15 or 20w derated?

anyway, open to thoughts/suggestions/criticism (apologies for lack of neatness, i work best with pen and paper)

[PRR]

How do you figure the gain of that amp?

[thetragichero]

i had some difficulty finding a resistance-coupled amplifier chart for 6sj7 but i probably just needed to scroll down more, so i just grabbed numbers from forum posts :-X (http://www.tubezone.net/pdf/rcachart.pdf page 11)
i should probably use Rp = 470k (close enough to 0.5M), Rg1 = 1M, Rg2 = 2M2, Rk = 1k5 eh? avoid the precisionvalue resistors and get close enough for rock and roll?
this is why i like another set of eyes, sometimes my pen gets ahead of my calculator/datasheet-fu

[PRR]

I just saw that you carefully biased your cathodyne to 1/3, then added a tone network which leaks DC. 2 ways.

I think the 2nd stage could be a 12AU7 gain stage. You probably need the gain (try without and see). The 12AU7 can easily drive volume and tone networks. The B side can be a cathodyne.

Are healthy 25Z6 still around? They were never common, and didn't last forever.

6J7 is exactly the same as 6SJ7 except the cap and pinout. Actually the no-S version is reputed to be better made. Also a 6J7/6SJ7 triode-strapped is exactly the same as 6J5/6SJ5, octal general purpose triode, a loose prototype for the mini-twin 12AU7.

[uki]

This site for tube data: https://frank.pocnet.net/sheets66.html

[thetragichero]

there are a few sellers on ebay that have new (testing) in box 25z6gt. just received one with a cracked base that i returned (might it still work? sure, but i'd rather not test my luck).
i have several organ pull 6sj7/6sj7gt (and one i purchased), plus a 6j7 pulled from an old test oscillator.
hadn't thought about using the second stage for gain, but i suppose it makes some sense.... i approached this partially from a build in which i used 12au6 preamp 12ax7 long tail pair and 2x12v6gt (transformer only had 12v secondary) that only has the vox cut control for tone and sounds very nice. i'm liking the sound of pentodes
i lifted the tone control pretty much directly from the supro thunderbolt... would a cap before the phase inverter prevent and dc leakage from messing with my phase inverter bias?

[sluckey]

would a cap before the phase inverter prevent and dc leakage from messing with my phase inverter bias?

Yes. And it will also keep that B+ voltage off your bass and treble pots.

[PRR]

Turned out that 25Z6 is readily available from web-order shops (not auctions) around six bux. There's also a 5-pin version, 25Z5, introduced April 1933. Cover story!
https://www.rsp-italy.it/Electronics/Magazines/Radio-Craft/_contents/Radio-Craft%201933%2004.pdf
6.66MB PDF

[thetragichero]

Yes. And it will also keep that B+ voltage off your bass and treble pots.

that's a good catch can't believe i missed it!

[PRR]

> keep that B+ voltage off your bass and treble pots.

Eh. They will probably survive. Just leave town before you turn the pots. It might be loud.

[thetragichero]

finally got a chance to scan version 2 of the schematic (was at in-laws' and they do not have a computer)
sort of in a holding pattern with this project while other life draws my energy, but have spent some time gutting things that i don't need for guitar amping (there's a lot of heavy metal mechanical items in there!) although i am trying to keep the reel-to-reel aesthetic as it looks really cool!

[thetragichero]

got this together and everything is working but the tremolo and i am stumped why my wiggler isn't wiggling

supply node voltage 152v
oscillator plate 25v (seems very low!)
oscillator grid -740mv
oscillator cathode 36mv
cathode follower grid 25v (connected to oscillator plate)
cathode follower plate 152v
cathode follower cathode 25v

what am i missing?

[tubeswell]

wiggler isn't wiggling

supply node voltage 152v
oscillator plate 25v (seems very low!

what am i missing?

The tube is conducting hard . Did you leave out a bias resistor?

[thetragichero]

would explain the almost non-existent cathode voltage but i've taken continuity measurements from the cathode resistor on my eyelet board to the socket and that measures good. the 'bypass' led (well, si diode and led in series to give me my desired forward voltage) are off board i suppose i should try disconnecting that and replacing with a more traditional bypass capacitor? (i was hoping to get the flashing led because that would look dope but i also just want the durn thing to work)

[shooter]

maybe check the Resistors in case you dropped a couple zeros, like 1M is really 1k kinda thing

[sluckey]

supply node voltage 152v
oscillator plate 25v (seems very low!)
oscillator grid -740mv
oscillator cathode 36mv

152V may not be enough for the oscillator circuit to work. The lowest supply voltage I've used is about 330V.

36mV on the cathode means the tube is only barely conducting, certainly not enough to oscillate. You should have about 1.75V to 2V on the cathode. I bet the LED is dark. You don't need the 4.7K across the LED. And you don't need a SI diode in series with the LED either. If you want a LED then that's all you need connected to the cathode.

[thetragichero]

152V may not be enough for the oscillator circuit to work. The lowest supply voltage I've used is about 330V.

this is my fear as well
i'm not averse to using sand if that'll get er a-wiggling. could a mosfet operate under such conditions? full disclosure: most of my experience with transistors is making them fuzz up. i see some interesting resources here, on the amp garage, and on geofex

[sluckey]

Your schematic is wrong! Install the missing cap. Then either remove the 4K7 cathode resistor and SI diode, leaving only the LED connected to the cathode, ***OR***, replace the LED/SI diode with a 22µF cap. I prefer using the LED.

[thetragichero]

i have that cap on the board but somehow neglected it on the schematic. i will attack the cathode resistor/led situation though when i go back out to the shop tomorrow

[thetragichero]

okay ripped out everything from the cathode, replaced with a single led. i know measure 1.25vdc on the cathode but no led illumination my friggin plate still measures 25vdc. i suppose next step is remove tube and check voltage on the plate again, potentially suspect a leaky cap between the plate and 3M speed pot?

[sluckey]

my friggin plate still measures 25vdc. i suppose next step is remove tube and check voltage on the plate again, potentially suspect a leaky cap between the plate and 3M speed pot?

You only had 152V to begin with. 152-25=127V dropped across that 470K resistor. That amounts to only .27mA flowing through the 470K, the tube, and the LED. Not nearly enough to light up the LED and not enough to make the oscillator work. You need more B+.

What tube is in the oscillator circuit?

[thetragichero]

12ax7.
also have a stash of 12au7, 12at7, 12av7, 12bh7 (would have to rewire for 12v heater since it's 300ma series string and change heater dropping resistor) and i have both 12dw7 and the very nifty jj reverse 12dw7. might have other noval oddities pulled from organs/test equipment. also need to get a tayda order going at some point so i could throw some power mosfets if they'd work better with the low B+