• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Eico HF-20 6B4G or other Triode Conversion

bandtuber

bandtuber

AK Subscriber
Subscriber
Hi all,
I have four Eico HF-20 amplifiers in various states of condition and functionality.
Two of the units work very well, and I have replaced caps where needed to ensure long term functionality. The other two amps have multiple issues, ground hum, bad soldering, etc and just are not worth the time to rebuild. I want to salvage the output and power transformers off of them and build a dual mono push pull 6B4G triode stereo amplifier. I would probably design on a new chassis that would allow me to use octal input tubes instead of annoying 9-pin where the sockets constantly have issues. Does anyone have a schematic that would work with the Eico 32004 OPT that is 7.5K primary. Other triode tubes can be considered, it does not have to be 6B4G if anyone has a better idea.

Thanks!
 
Register to hide this ad
If you use a choke input supply to get the voltage down into the 330ish volt range you would have a good chance with 6B4 or 2A3. 280v B+ and 50-55v cathode bias give a decent operating point. The HF-20 only has a single 6.3V filament winding so you will need a separate heater transformer for the output tubes. I have a PP 6B4G amp which has B+ of about 300V and somewhere around 45-50V cathode bias. With well matched power tubes I don't have perceivable noise using a center tapped filament transformer with the cathode bias resistor connected to the CT. Depending on your speakers you might want DC for the heaters if you have high sensitivity models.

Here is an example operating point.
1792509724171.png

My amp has a 5k output transformer so a little different op point. The grid chokes aren't a necessity but I wanted to try them at the time I built the breadboard.

6B4G DIY amp complete.jpg
 
Last edited:
Thank you!
Still deciding on what to do- I got super focused last night and got one of the basket case units running...
 
Some years ago, I used those transformers to design/build some really wonderful push-pull fixed bias 6550 triode amplifiers into a couple of old HF-60 chassis I had. Front end was standard Williamson with a 12AU7 residing in the EF86 socket. 20 watts RMS 20Hz - 20kHz with very low distortion. Wonderful amps, and excellent transformers, although they need a dual feedback loop from the secondary for best results.

Dave
 
400 Volts DC on a 6B4 plate is pushing the tube very hard. It can be done with -90 V bias, though. The tubes will run hot to the touch; not a good scenario for long term usage.

I also recommend using a choke input power supply as @nerdorama advises. Perhaps, a more appropriate power trans can be found. Dual 6.3V windings, will make it easier to cathode bias and null out any hum. Each pair of opt tubes would use their own winding; one can also furnish filaments for the front end tubes.

Should you choose fixed bias, the HF-20 opt trans is not the best choice. Fixed bias 2A3, 6A3, 6B4 type tubes like 3K plate to plate for fixed bias, 5K p to p for cathode bias. VT52 triodes can run on 6.3V filaments and would like the 6.6K opt trans of the HF-20.

A larger chassis will certainly help with cooling, compared with the HF20 chassis.

So, what type of front end are you planning to implement ? Drive is important for triodes.
My favorite has always been using "Single Plate to Push-Pull" interstage driver transformers. They are not easy to source, but they are out there. The speed, depth and soundstage provided by the driver trans is unbeatable; probably the finest phase inverters ever. The "reach in and touch the instruments" soundstage is very realistic.

The interstage matches impedance, gain and lowers the number of tubes needed to drive the triode outputs.

Another fine triode is the octal 6A5G, which is an indirectly heated triode, looking like a 6B4G. Peerless used the 6A5G for their obscure, Peerless A-100A amps in 1950, to compete with Brook. They used a 10W push-pull ouput transformer (with unused secondaries), as a Center Tapped Choke within their low impedance driver circuit.
These rare amps inspired me to build another pair, from scratch, to compete with my own No Negative Feedback, Interstage Driver equipped, PP Triode amps.

Enjoy building your triode amps. Know that there are many of us, within AK, that share your plans for higher fi...
 
Last edited:
maybe something slightly off the standard path, but 6BX7 with the sections paralleled might work out. Similar plate resistance if paralleled, bit lower dissipation though. 12 watts total for both sections. They are rated for higher voltage than a 6B4 too so possibly more compatible with the existing power supply.
 
For the record, the Eico HF-20/22 OPTs are 7800 Ohm transformers, with the very earliest examples (that have no identification markings on them) reaching 8000 Ohms.

Dave
 
Oh...Thanks Dave ! 7.8K or 8K is definitely not optimum for the 2A3, 6A3, 6B4 family of tubes. 45s, 50s, VT52s would be fine, though.
Strong 50s and VT52s can run on 6.3V filaments, nicely.

Has anyone measured primary inductance of the 32004 opt, as well as leakage inductance ? A decent LCR meter can measure that, easily...
 
You must log in or register to reply here.
Back
Top Bottom