ST-35 with Compactrons

[TerrySmith]

Here's a ST-35 built using parts I had laying around. It sounds exactly as a stock '35 should!

The tubes are 5HC7/5HA7 compactrons, 29GK6's for the outputs, and a pair of 6AX3 dampers for the rectifier.

Transformers are Hammond 270HX and 1650F's from Shannons basement. The gray trans is a Rat Shack 25.2v unit i've had since I was a kid, added some end bells from the junk box to make it look normal. It lights the 29GK6's.

The chassis was a damaged part due to AES poorly packing it, they did make good on it.

[Geek]

I love it!

It's always nice to see such "useless TV tubes" being put to good use Yellow_Light_Colorz_PDT_16

Cheers!

[WA4SWJ]

Terry,

Very, very nice!!

Excellent work.

[EWBrown]

Excellent, work! and accomplished in the true spirit of "the plinkertron" Yellow_Light_Colorz_PDT_03

I like the Tung Sol 5HC7s and 4HC7s, as they have the ribbed gray plate innards, and they are spaced apart rather than crammed together, inside the tube. Back a few years ago, when AES was unloading these tubes at 50 cents each, I pretty much cleaned out their inventory


As I have only one 29GK6, but lots of 8BQ5s and 10BQ5s:

I've been working up a similar concept, using 4HC7s (filaments wired in series) and 8BQ5s in an ST-35 design. I have plenty of those tubes, gathered over the years...

I've also been playing around with 5HA7 / 5HC7 VA / CF driver designs on Tube CAD, as well, got LOTS of these tubes and I need to put them to good use...

In a nutshell, 300V B+, run the gain stage at 1 mA, RA = 150K, RK = 1200 ohms, bypass with 47 uF, 10V. Direct couple to the CF, using 27K, 2W cathode resistor, this sets its current around 5.75 mA, 2.2 output cap, and 100K across the output (or use it as the power tube's grid resistor).

The overall voltage gain is about 60X, and the maximum output "swing" is +/- 80V peak (160V PK-PK) . Should be sufficient drive for 2A3, 6B4G, 300B and triode strapped horiz sweep tubes.

What is the "long chassis" amp in the background?

/ed B

[TerrySmith]

Thanks for the compliments!

I have many 5HA7 and a few 5HC7 tubes I got at the AES 25 or 50 cent sale. Ed, that's a good idea on the 4HC7/ 8BQ5 combo, as I have a few 8BQ5 RCA's. I SWAG you could run your 6.3v into a FWB then to a 3300uf cap and have the 8 volts there.

The amp in the background is a mutt, built around the Diy ST-70 board, el84's, and Heathkit AA-100 OPT's. the rectifier is a 5BC3, a novar version of a 5U4.

[mesherm]

Nice job Terry!!
Now if only they made "plinker" transformers... Yellow_Light_Colorz_PDT_16

[Brik]

Beautiful craftsmanship!
I love the originality, too.
Please keep posting more of your work. Yellow_Light_Colorz_PDT_12

/b

[Geek]

Beautiful craftsmanship!
I love the originality, too.

DAT's DA VERD I VUZ LOOKINK FOR! Yellow_Light_Colorz_PDT_04

Very origional :)

Cheers!

[EWBrown]

8BQ5s are especially flexible, the filaments are rated as 8V, +/- 0.7V, so there is plenty of allowable latitude.

Another source of approximately 8V for the filaments is to get ahold of one of the 1970s vintage 5V, 3A linear regulated power supplies, the power trannie and rectifiers usually deliver around 8VDC before it hits the regulator and pass transistor. I've seen these PSUs for dirt cheap, and even if the regulator is blown, the trannie is almost never bad.

FWIW, a 6K11/6Q11 can serve as a 6.3V filament 5HA7. Just use triodes #1 and #3, the pinouts are compatible, and triode #2 is not usesd (pins 5, 6, and 7). The high gain ald low gain sections are also compatible.

6U10, the low gain and high gain sections are swapped around, so that one is NOT a drop in replacement.

6AC10 is almost three 12AT7 sections, close enough for most 12AT7 VA and LTPI (as in VTA-70 driver) applications.

Another of my many incomplete or uncompleted amp projects is a PPP EL84, using the Poseidons, or posswibly a VTA-70 driver board as the drivers. Basically a rip-off of the VTL "tiny triode" 25W triode, 46W UL design, but as a single chassis "dual mono" design, rather than two monoblocks.

The related project is a stripped Hhath AA-151, retaining only the original bare chassis, traisformers, basic power spply and power amp sections, using Poseidons or equivalent P2P circuits as the drivers. That one is much closer to completion, I have 80% of it functional. Just to choose the final VA/LTPI design remains....

/ed B

[TerrySmith]

An idea popped in my head the other day so I thought I would try it. I removed the 25v transformer and the 29GK6 tubes, rewired the filament circuit as pictured, and installed NOS RCA 10GK6 tubes. These 10GK6's are identical in construction to the RCA 6BQ5 except for pinout and filament ratings. 10BQ5 tubes would work also. The purpose of this mod is to power oddball tubes with an off-the-shelf transformer, a Hammond 270HX in this case, and not have any extra filament transformers.

Still sounds excellent like it should, no extra hum or noise.