I am looking for info on building an integrated tube amplifier with off the shelf components, to used in the build. I have been told to look out for the RA-100 as any one any info on this amp?
I've got one built up from the parts that Fred had left behind,I guess he was planning to build a second "RA-100a" version. I have moved house since building it and haven't had time to pull it out in a while. I still need to set up a proper bench too.
http://www.diyaudio.com/forums/tubes-valves/108490-eh-el34s-600v.html
RA-100 Amp pictures by Nothing40 - Photobucket
If you're interested,I can help you out with some of the parts I used,etc. I think I still have a few of the PCB's left also.
One suggestion would be to use a lower voltage power transformer,as you can see the B+ voltage is rather high! ~640Volts in my case.
A while back I had asked Edcor for a quote on a custom power transformer,and it was very reasonable(~$120,IIRC),so I think I'll probably go that route eventually.
One other note,with such a high B+ voltage I had to change the +230V regulator a little. I replaced the bipolar transistor(It was right on the ragged edge of SOA.) with a high voltage FET,and increased the value of the resistor on the zener chain.The zeners would drift as they heated up,and lowering the current through them helped cool them off and keep the voltage more stable.
I changed the pass transistor on the negative (-590V) rail also..I don't remember the part number offhand.The NTE38 that Fred used was hard to find,so I sub'ed in a suitable transistor..I'll find the number if you are curious.
http://www.diyaudio.com/forums/tubes-valves/108490-eh-el34s-600v.html
RA-100 Amp pictures by Nothing40 - Photobucket
If you're interested,I can help you out with some of the parts I used,etc. I think I still have a few of the PCB's left also.
One suggestion would be to use a lower voltage power transformer,as you can see the B+ voltage is rather high! ~640Volts in my case.
A while back I had asked Edcor for a quote on a custom power transformer,and it was very reasonable(~$120,IIRC),so I think I'll probably go that route eventually.
One other note,with such a high B+ voltage I had to change the +230V regulator a little. I replaced the bipolar transistor(It was right on the ragged edge of SOA.) with a high voltage FET,and increased the value of the resistor on the zener chain.The zeners would drift as they heated up,and lowering the current through them helped cool them off and keep the voltage more stable.
I changed the pass transistor on the negative (-590V) rail also..I don't remember the part number offhand.The NTE38 that Fred used was hard to find,so I sub'ed in a suitable transistor..I'll find the number if you are curious.
Looking for advice on RA 100 tube amp (F. Nachbaur)
RA 100 tube amp by Fred Nachbaur
Hi. I saw the posts and wondered if you were still around and could offer any advice.
I have built the main parts of the RA 100 and have a problem with little/no amplification with no neg feedback and really loud feedback when the RC feedback loop is connected. All my voltage readings are fine and I can get the 50mA cathode current set on all four 6L6GCs, but I have some hum from the mains transformer when the B+ is switched on as if it is being loaded too much. I can adjust the hum out, but there is liitle cathode current by then. I'm getting a similar B+ to you under load and I know this is a little high for the tubes' spec, but neither Fred (designer) nor you seem to have a real issue with this. There must be another problem with my circuit(s) somewhere. I am in the UK and a guy from the vinylengine site has been helping me via email from California, but I'm not sure if he has reached his limit of help as he is suggesting trying a completely different circuit. As I have invested loads of time and around 800GBP in parts for this amp I do not want to stop now. For someone with more experience than me it may be a simple fix.
Can you offer any advice, or did you have any similar problems when building yours?
Any help/advice would be grstefully appreciated.
George
RA 100 tube amp by Fred Nachbaur
Hi. I saw the posts and wondered if you were still around and could offer any advice.
I have built the main parts of the RA 100 and have a problem with little/no amplification with no neg feedback and really loud feedback when the RC feedback loop is connected. All my voltage readings are fine and I can get the 50mA cathode current set on all four 6L6GCs, but I have some hum from the mains transformer when the B+ is switched on as if it is being loaded too much. I can adjust the hum out, but there is liitle cathode current by then. I'm getting a similar B+ to you under load and I know this is a little high for the tubes' spec, but neither Fred (designer) nor you seem to have a real issue with this. There must be another problem with my circuit(s) somewhere. I am in the UK and a guy from the vinylengine site has been helping me via email from California, but I'm not sure if he has reached his limit of help as he is suggesting trying a completely different circuit. As I have invested loads of time and around 800GBP in parts for this amp I do not want to stop now. For someone with more experience than me it may be a simple fix.
Can you offer any advice, or did you have any similar problems when building yours?
Any help/advice would be grstefully appreciated.
George
http://www.dogstar.dantimax.dk/tubestuf/graphics/pwramp-l.gif
What do you mean with loud feedback?
Did you perhaps make a mistake with the connection of the output transformer?
What do you mean with loud feedback?
Did you perhaps make a mistake with the connection of the output transformer?
Last edited:
I have checked the output transformer connections and they seem fine. I do get some sound from the speakers, but it is very small.
The 'feedback' noise results when the loop from the speaker to the input of the 12AT7 is connected. It is deafening and once blew the cathode resistor on one 6L6GC. I am using 1650NA output transformers that have a simpler output connection, ie. one wire for each different speaker impedance. I have 8 ohm main speakers, but for testing I'm using 6 ohm ones and the yellow 8 ohm output tap. There is a 4 ohm one (green) but I haven't tried that yet.
The 'feedback' noise results when the loop from the speaker to the input of the 12AT7 is connected. It is deafening and once blew the cathode resistor on one 6L6GC. I am using 1650NA output transformers that have a simpler output connection, ie. one wire for each different speaker impedance. I have 8 ohm main speakers, but for testing I'm using 6 ohm ones and the yellow 8 ohm output tap. There is a 4 ohm one (green) but I haven't tried that yet.
This is a reply to this message in my normal email:"The current through the driver (12AT7) is essential to the amplification.
What voltages are on their anodes?"
I can adjust to any voltage in the range suggested by the designer (and beyond!). I set them all to -72V, but when I adjust to get, say, 45mA cathode current the voltages go down to around -60V.
What voltages are on their anodes?"
I can adjust to any voltage in the range suggested by the designer (and beyond!). I set them all to -72V, but when I adjust to get, say, 45mA cathode current the voltages go down to around -60V.
The point is that the PRIMARY leads are incorrect so that you have positive feedback instead of negative feedback. Change the primary leads so that each lead goes to the opposite output tube. There may be other faults too, but you need to correct this first.
Good tip on the OPT wiring.
Also,more build info,etc. can be found here:
RA-100 Build
RA-100A layout
Also,more build info,etc. can be found here:
RA-100 Build
RA-100A layout
Thanks for the replies and links; I've had a good read through them both and taken mental notes.
The rewiring of the OPTs worked a treat: no more feedback noise. Thanks for that tip. Now I just need a 'simple' fix to get the 'Beast' (as I have named it, partly due to its weight, but also the struggle I've had getting all the sections made and working) to actually amplify the signal from my homemade sinewave generator. I had the usual teething troubles; some arcing across adjacent tracks on stripboard I've used for some of the power supply. Blown/burnt resistors, transistors and an exploding electrolytic cap due to excess voltage, other caps venting or with the top bowed out. But all these are behind me now I trust. Components have been replaced, etc. You guys in the US are lucky re. cost of transformers. We can get the Hammonds here in the UK, but I've spent over 350GBP on the 3 transformers and choke in total (without carriage)
Looking forward to hearing some real music from it.
George
The rewiring of the OPTs worked a treat: no more feedback noise. Thanks for that tip. Now I just need a 'simple' fix to get the 'Beast' (as I have named it, partly due to its weight, but also the struggle I've had getting all the sections made and working) to actually amplify the signal from my homemade sinewave generator. I had the usual teething troubles; some arcing across adjacent tracks on stripboard I've used for some of the power supply. Blown/burnt resistors, transistors and an exploding electrolytic cap due to excess voltage, other caps venting or with the top bowed out. But all these are behind me now I trust. Components have been replaced, etc. You guys in the US are lucky re. cost of transformers. We can get the Hammonds here in the UK, but I've spent over 350GBP on the 3 transformers and choke in total (without carriage)
Looking forward to hearing some real music from it.
George
ThANKS AGAIN FOR THE HELP AND INFO SO FAR. I've had a little break from 'the beast' as I have entitled my RA 100. I still have a need for help if anyone can help further with likely problem(s) that are still present.
Many thanks in advance
George
Many thanks in advance
George
Just wondering if there are any comments or tips on building/testing the preamp board as an electronic engineer friend has identified this as not working properly. I guess as some of you have built this successfully the preamp board circuit is fine, but I have a problem with little amplification and an unresponsive volume control until the last quarter of its travel.
Here's hoping I can get it working sometime soon
George
Here's hoping I can get it working sometime soon
George
- Status
- Not open for further replies.