Rosenblit 70w Mac amp

rubofdagreen · 22nd June 2010, 12:55 AM

I have rebuilt a pair of Glass audio projects from 1990. They worked fine until I fixed them. One amp makes a high pitch 1000 maybe 1500hz note when i try to operate it on the speaker. Works very well with an 8ohm resistor attached with the speaker, (parallel). I tried hooking them in series and it makes the noise. Tried switching out the driver tubes with the ones in the amp that is fine, no. Any ideas, it's driving me crazy. Thanking you in advance.

atmasphere · 22nd June 2010, 01:37 AM

Did you do anything that might have reversed the phase of the feedback? Swapped plate connections on the OPT for example?

rubofdagreen · 22nd June 2010, 02:59 AM

I tried switching the leads for the plates on the output tubes earlier. That was not a good idea, oscillated like mad. I just took out the coupling caps and tested them on my handy dandy 4262a lcr meter, they tested fine. Replaced them and now a resistor in the voltage regulator that drops the voltage going to the plates of the phase inverter fried. I had the known good tube from the working amp in there. I at least have a place to start looking now. Thank you.

22nd June 2010, 12:55 AM	#1
rubofdagreen diyAudio Member Join Date: Jan 2010	Rosenblit 70w Mac amp I have rebuilt a pair of Glass audio projects from 1990. They worked fine until I fixed them. One amp makes a high pitch 1000 maybe 1500hz note when i try to operate it on the speaker. Works very well with an 8ohm resistor attached with the speaker, (parallel). I tried hooking them in series and it makes the noise. Tried switching out the driver tubes with the ones in the amp that is fine, no. Any ideas, it's driving me crazy. Thanking you in advance.

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22nd June 2010, 01:37 AM	#2
atmasphere diyAudio Member Join Date: May 2007	Did you do anything that might have reversed the phase of the feedback? Swapped plate connections on the OPT for example?

22nd June 2010, 02:59 AM	#3
rubofdagreen diyAudio Member Join Date: Jan 2010	I tried switching the leads for the plates on the output tubes earlier. That was not a good idea, oscillated like mad. I just took out the coupling caps and tested them on my handy dandy 4262a lcr meter, they tested fine. Replaced them and now a resistor in the voltage regulator that drops the voltage going to the plates of the phase inverter fried. I had the known good tube from the working amp in there. I at least have a place to start looking now. Thank you.

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