Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: grovesyblues on June 13, 2023, 12:23:54 pm
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I need some help and advise please, my 35 Year plus Audio Innovations S1000 preamp had a problem with the 6SN7 DC heater circuit, the heater voltage was dropping off when the unit got hot. As it is so old and the problem was intermittent, I decided to replace all the components in the circuit. I had a circuit diagram of the unit (attached is the heater circuit). However it seems the diagram I had was not as built (I have called that circuit MK 1), I drew out the circuit before starting the work, I'm not sure I exactly follow the circuit but I am sure I got it right (The circuit marked MK2). However when I power the unit up with no Valves (Tubes) installed across pins 7 & 8 I see 16v instead of the required 6.3v, this seems way too high. I'm not sure I understand the circuit as one side of the transformer secondary goes to the centre tap of the two capacitors, there is no connection to the the other AC side of the bridge. I am wondering if this is a high voltage off load or if there is something else badly wrong. I took photo's of how it was built before I started and I am sure I have rebuilt the circuit exactly as it was; what I did not do was measure the voltage off load with the original circuit, big mistake. Can anyone provide any advise please.
Many thanks
Simon
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Without the tube you have no path for current to flow. No current flow means no voltage drop.
Plug the tube in and things should be happy.
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Thank you for the reply, I am a little concerned tat as the voltage was so high I might damage the tubes. I did wonder if when current flows it will be OK but I did not want to take out my 6SN7 tubes. If the consensus is that this should be OK and the circuit looks good, I will give it a try tomorrow.
Many thanks for taking the time to reply :smiley:
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If you are afraid to put the 6SN7 in the socket, then plug in a 10Ω/5W resistor between pins 7 and 8 to simulate the 6SN7 filament. Measure the voltage across the resistor leads. You should see that voltage drop down considerably.
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I put in an old tube today and gave it a try. I have to admit I don't understand how this circuit works, I can not find anything similar in my searches. Could anyone kindly explain what is going on here and why the designer has done this? The circuit feeds 2 x 6SN7 valves in the final stage of a phono preamp. Prior to these tubes there are ECC83's fed by a different heater circuit, also shown on the attached circuit, that makes sense, but it's the 6SN7 heater circuit that I don't understand.
With one of the 6SN7's in Circuit the voltage dropped to 9.5v DC, the heater seem very bright, I put the second 6SN7 Tube in circuit and the voltage dropped to exactly 6.3v DC and the heaters looked fine. Looks like the original designer got this spot on for two Tubes, not a good idea to run with just one, but no reason to as it's a Stereo Phono Stage. The power supply is also a very interesting design, but that is for another day.
I think I will call this repaired for now, many thanks for all replies. If anyone has any thoughts on how this circuit works, I would love to know.
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The Mark I circuit is a standard FWB circuit. The 270Ω (sure about that value?) is in series with the tube filaments.
The Mark II circuit is only using two of the diodes in that FWB brick. They are connected in a voltage doubler circuit. The 2.2Ω is in series with the tube filaments.
Which circuit is in your amp?
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My unit has the MK2 circuit, I had to draw it out from what I found as built. This is a very rare unit from UK company Audio Innovations, they didn't make many it seems, there is nothing about this unit on the web. It's only a phono stage with RIAA for MM, I use an MC with a step up transformer, the end power amp is a 300B design. The whole system is great but getting old like me now :laugh:
So the circuit is a Delon doubler which explains the 16v off load, it does give 6.3v with the two 6SN7's installed. You learn something every day.
The MK1 circuit is their drawings that I asked for when I bought the unit in 1985, so the 270 ohms is what they said, there is an earth symbol on the positive side that I think is a mistake! I added the MK2 from what I found in my unit.
Thank you for your great answers and support :m11