I bought one of these mainly because I can;t actually buy transformers and a chassis for that much here in oz.
On googling about for a schematic and or datasheets, I can;t find anything about this FU32 tube. I certainly found GU-32 datasheets, but that's about it.
Can anyone shed some light on what they are exactly? Thanks in advance!
On googling about for a schematic and or datasheets, I can;t find anything about this FU32 tube. I certainly found GU-32 datasheets, but that's about it.
Can anyone shed some light on what they are exactly? Thanks in advance!
It will have to be for 1.5w or less. The OPT will begin to Saturate at 1.3W or so at 50hz. Other then that it is a sweet amp once you get the bias right.
I tried a inexpensive PCB based kit with FU32 and 6J1. Distortion is comparatively high and detail is lacking; I am wondering if the kit on this post does use the same schematic (I attach mine). The supplied tiny 5K output transformers are rated 5W but I believe that 3W is a more accurate specification. I tested them with a triode connected EL84, they haven't any mayor issues.
Right out of the box, B+ was 240V. I measured 22,5V at the FU32 cathode - this means about 100mA of total cathode current. After a while the FU32 plates had a hint of red; the cheaply designed switched mode power supply was overloaded/overheated and started to behave erraticaly. I managed to get proper operation of the power supply with some modifications on its filter and feedback loop. B+ is 265V now and the cathode resistor is 268 ohm, this means a total cathode current of 90 mA (45 mA each section). Sound quality did not improved. I tried with other 6J1 tubes and with a RCA 832 tube, no change. I now believe that the 6j1 gain stage on my kit is biased in a sub optimal way, and I was about to start drawing a load line on the attached 6ak5w triode curve to check it out (assuming that is close enough to 6J1). It may also be that 45mA is still too much for the output transformers. My target is at least 1W output, anything more is welcomed but not required. Any suggestion is welcome.
Right out of the box, B+ was 240V. I measured 22,5V at the FU32 cathode - this means about 100mA of total cathode current. After a while the FU32 plates had a hint of red; the cheaply designed switched mode power supply was overloaded/overheated and started to behave erraticaly. I managed to get proper operation of the power supply with some modifications on its filter and feedback loop. B+ is 265V now and the cathode resistor is 268 ohm, this means a total cathode current of 90 mA (45 mA each section). Sound quality did not improved. I tried with other 6J1 tubes and with a RCA 832 tube, no change. I now believe that the 6j1 gain stage on my kit is biased in a sub optimal way, and I was about to start drawing a load line on the attached 6ak5w triode curve to check it out (assuming that is close enough to 6J1). It may also be that 45mA is still too much for the output transformers. My target is at least 1W output, anything more is welcomed but not required. Any suggestion is welcome.
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very nice old ( ww2 era ) transmitter tube - here's my gu32 amp based on audiohobbyist circuit (crc psu - tubes from 1963 - many oh them have lost their vacuum..
+ nice 6h23p ). Very sweet sounding amp ! and simple. Grid stoppers are essential - without them you will have oscillation - its transmitter tube after all .
+ nice 6h23p ). Very sweet sounding amp ! and simple. Grid stoppers are essential - without them you will have oscillation - its transmitter tube after all .Attachments
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tnx,
Of course, it is not easy to work in such a small chassis ...I use 20 W pp OPT 10K :8R and for tube heater 6.3v i use fake center tapping to ground - with two 150R resistors ( 1% tolerance) - silent amp - no hum.
gate stoppers should be at least 470R according to my experience (the oscillation may be inaudible - but there will be higher H distortions visible in FFT)
Of course, it is not easy to work in such a small chassis ...I use 20 W pp OPT 10K :8R and for tube heater 6.3v i use fake center tapping to ground - with two 150R resistors ( 1% tolerance) - silent amp - no hum.
gate stoppers should be at least 470R according to my experience (the oscillation may be inaudible - but there will be higher H distortions visible in FFT)
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aspringv,
The schematic (post # 15 and 16) looks good. Here are some important things to consider:
There will be poor left / right channel separation. The 2k Ohm screen resistor does not have a bypass capacitor to ground, it needs one. The screen voltage will vary with L and R signals, putting those signals onto the opposite channel.
This tube has a common cathode, common screen, and can work up to 300MHz. The plates are tied to the output transformer leakage reactance (high frequency inductance). You may have oscillations with different speakers, and different signals. You ought to have 470 or 1k Ohm grid stopper resistors on each control grid.
Those tubes have a maximum control grid resistor rating of 50k Ohms (usually in fixed bias operation). But you have 300k grid resistors. The only thing saving the tube from thermal runaway is the 330 Ohm cathode self bias resistor. Put some loud music on that drives the tube slightly into clipping (I know you do not listen that way, but it may drive the cathode current up a little . . . Test to see if that causes thermal runaway.
Make sure there is always a load on the outputs.
It should be a sweet sounding amplifier. Have fun listening.
The schematic (post # 15 and 16) looks good. Here are some important things to consider:
There will be poor left / right channel separation. The 2k Ohm screen resistor does not have a bypass capacitor to ground, it needs one. The screen voltage will vary with L and R signals, putting those signals onto the opposite channel.
This tube has a common cathode, common screen, and can work up to 300MHz. The plates are tied to the output transformer leakage reactance (high frequency inductance). You may have oscillations with different speakers, and different signals. You ought to have 470 or 1k Ohm grid stopper resistors on each control grid.
Those tubes have a maximum control grid resistor rating of 50k Ohms (usually in fixed bias operation). But you have 300k grid resistors. The only thing saving the tube from thermal runaway is the 330 Ohm cathode self bias resistor. Put some loud music on that drives the tube slightly into clipping (I know you do not listen that way, but it may drive the cathode current up a little . . . Test to see if that causes thermal runaway.
Make sure there is always a load on the outputs.
It should be a sweet sounding amplifier. Have fun listening.
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Thanks! Thats some useful info. From playing with 'fets it's a given there that gate stoppers are included. I'm often surprised how many scematics I see without (I've another cheapie chinese amp kit here with the same omission).
I'll knock it together when It gets here and report back.
Cheers!
You may find it interesting that this configuration does function safely and the results of a sound impeccable will satisfy you. As for that Chinese kit .... $ 90 has the price and ... $ 10 does everything, we can not get a quality exceptional with only $ 90, to be serious, I think it's a joke!
Careful! good results are only obtained with an output transformer with Ra of 6500 ohms!
Careful! good results are only obtained with an output transformer with Ra of 6500 ohms!
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