Jose Martins sent me an email with his recent built on the 27/56 preamplifier using the gyrator load and these PCBs. I recently posted an idea using the lovely 27 IHT valve here.
Here is a picture of the finished preamplifier:
Hello Ale,
Here is some photos of my new creation where I used your Gyrators boards.
This is a line 27/56 preamplifier. Working great with your Gyrator boards. I’m planning to use also in near future a second output with Magnequest parafeed transformers B7/15K.
I built this project in old Antique Sound Lab AQ2001 inox steel chassis that I have for some years in my garage.
The filament supply for tubes are regulated and noise of this preamplifier is very low even with old Arcturus 127 triodes from 1923. It is very important to connect the cathode of these old tubes to the filament.
I can also use 56 triodes to get more gain with no modification. Now I must test all my stock of 27 and 56 triodes. I’m very happy with the sound even compared with others DHT tube preamplifiers I have (26, 4P1L, VT-25, 1G4, RE084, Ba, Ca, TM2).
I’m very happy with your boards. Thank you for your great products.
Best Regards
José Martins
Ps: please excuse my bad English.
This preamp looks great, well don Jose. BTW: your English is really good, so nothing to apologise of. I hope to get some circuit diagrams and description from Jose to update the post. In the mean time, here are some detailed pictures of this fantastic build:
Update 1
Here are some further notes from Jose:
“The preamplifier is based in your 27 circuit, but I not used fixed bias with battery on the grid of the tubes, instead I used a auto bias with an 1.5KΩ resistor with the cathode bypassed cap. After tried several types of caps I chose the Sanyo OSCON 100uF /16V.
The volume pot is a Alps Black 40mm 50KΩ log directly to the grid of the tube.
For HT supply I used the original tube power supply from original ASL AQ2001, tube rectifier 5Y3 and tube regulated with 12AX7 and 6V6, with a few changes in the circuit and in type of filter caps. I think with the gyrators the power supply could be more simple but in this project i used what I have on hand.
In these old triodes it is crucial to connect the cathode directly to filament according to the instructions of data sheet. Without this connection the hum noise is very loud.
I decide to supply the filament with DC because I already have all the components to make the regulators and to reduce as much as I can the noise. First tried the current regulators but I got too much hot from regulators and power resistors and never have the right tension between different tubes, so finally I decide to built a voltage regulator. I don’t like voltage regulators but these are indirect tubes and the quality the filament supply don’t influence much the sound of these kind of tubes like in DHT tubes.
Now the hot generated by the chips (LT1084) are acceptable when used with 27 triodes (1,75A) for a long period. Besides that, I can easily change between the 27 and 56 with no need of any adjustment in the regulators. The ripple with these voltage regulators is negligible and contribute to a low noise preamplifier.
The sound is better than I expected from these old IHT tubes.
Another important aspect to be taken into account when using these old tubes is the bad contacts of the pins that make the tubes generate all kind of strange noises. After a good cleaning and re-solder all the pins these strange noises are gone.
Some very old tubes are microphonic and capture many external interferences, but these interferences can be attenuated or completely eliminated with a electrostatic shield. The michophonic in manageable with some care during the construction. In my case only four tubes from my stock of the oldest 27 triodes have high level of microphonic.
I adjusted the gyrators boards 5mA with 27 triodes and with 56 the plate current low a little bit to 4,5mA that is in line with the specs on the data sheet.
I tried this preamplifier with some power amplifiers (valve and transistor) and in all cases the sound is very detailed and musical with the pleasant and magical sound of these old triodes.
I recommend very much your boards and this circuit that give me great results. I have new ideas to use your boards again with other old tubes I have.
Best Regards
José Martins
Portugal”
Post updated with further circuit description from Jose!
Hi everyone! I’ve been lurking around but finally need to post something to hopefully revive this thread. I just finished my own 27 preamp, but I’m having a few problems: EMI and noise. The power supply is 6X4 LCLC. Total inductance is 30 henries, and total capacitance is around 110uF. The supply is pretty quiet, and I even elevated the 6.3V heater on the rec to 30VDC. Tubes are plate loaded with a CCS at 5ma, and cathode bias is -7.4V via series Vishay blue LEDs. Moving on, I have heated the tubes in series with a 5VDC switching PS, with its ground strapped to the amps ground. It was mentioned you must strap the cathode to the filament. Shall I assume this is what I have done, or am I missing something? I have a collection of the early globe meshplates, and for the life of me, am flummoxed by how much they seem to suffer from EMI. Not obnoxious, but enough to frazzle and frustrate. Anyway, just wanting to know if anyone has extra advice to try to quiet down one of these bottles! My next move is to AC heat these and DC elevate them. Thanks for any advice!