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The Plan
The Plan
Power amp will use 6V6 tubes with cathode bias. This is what the amp currently has. I can't put 500V on their plates, so I found a 250-0-250 power transformer on ebay for $11.
Add 2 sockets so the 6V6 tubes can be removed and EL84 tubes put in. Because EL84s need less drive than the 6V6s, I expect this will give the amp a more distorted character. I've seen output transformers advertised as being suitable for either 6V6 or EL84 tubes, so I think the impedance should be in the approximate range. I might go so far as to try and figure out what it is on this transformer.
Use a solid state rectifier, but put in Kevin O'Connor's sag control kit to allow any amount of sag to be dialed in. I used this kit on the Bassman 10 I had for a while. This amp used fixed bias, and when too much sag was dialed in, the B+ would drop and the bias would become colder. I think the sag control kit might be a better match for cathode bias which might adjust itself.
Build an octal preamp and phase inverter using 6SL7 tubes. The current PI is a 6SL7, and the 2 preamp tubes are octal 6SF5 metal tubes. I don't have an octal tube amp, and want to hear their sound. I understand they tend to be more microphonic, but otherwise not too much different in character from 12AX7 tubes. Because of the microphonic tendency, the preamp will only have two stages, and use one 6SL7 tube.
Build a second preamp with an EF86 tube. This is a pentode, and was used in the early Vox AC30/4 amp. The EF86 will be the only preamp tube and will connect to the 6SL7 phase inverter.
I haven't decided on the location or kind of tone controls yet. This might happen after I get the amp up and running.
By removing the vibrator power supply, I have a lot of room in the chassis, so I thought I'd build each of these sections on a separate circuit board. This will make modifying the circuit a little more flexible, since I can completely re-do the circuit board for a section without affecting the other sections.
This amp was being thrown out, so I grabbed it. It looks like it's an early Bogen (uses Bogen parts) or a Bogen spin-off company. It has 2 power supplies, one for 120VAC and one for 6VDC--so you can run it off the tractor for the barn dance PA system.The 6v supply uses mechanical vibrators to both change the DC to AC so it can be stepped up by the transformer and to rectify the stepped-up voltage.
I carefully powered it up using a Variac and in-series light bulb to limit the current. I wasn't sure whether it was designed for 110VAC or 120VAC. 120VAC provided 6.3V on the heater tap, so I assumed 120VAC design. Nothing seemed to be drawing too much current so I removed the lightbulb. The amp did make sound, but it was motorboating with lots of hum. The large silver can cap near the front of the amp was also slightly warm, so I subbed in some similar values. Still motorboats, so new caps for the silver can on the left side. That did it. This gave me a good clue that the transformers and tubes were OK.
I was able to get some voltage measurements before the 7Z4 rectifier tube blew. Afterwards I subbed some 1N007 diodes in for it, and checked the B+ with no tubes in--it was now above 500V. The 7Z4 rectifier tube can only supply 100mA of current, but it must also have a large voltage drop across it because the B+ I had measured with it in was about 380V. I would really have liked to be able to check that this was so, but not so much that I'd buy another 7Z4 tube. So using the Variac, I adjusted the B+ to 380V and made some other measurements. If this is an accurate test, the voltages on the preamp tubes were really low (<50V) when compared to current guitar amps.
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