Antique Radios Forum

This is the temporary home of the Antique Radio Forums while a new server to host it is being found. You will have to register here to post. Please Note: None of these topics will be preserved when the original forums are back.
 
HomeCalendarFAQSearchMemberlistUsergroupsRegisterLog in

Share | 
 

 GEC 912 PLUS amp

View previous topic View next topic Go down 
AuthorMessage
Tube Radio



Posts : 31
Join date : 2010-10-12

PostSubject: GEC 912 PLUS amp   Thu Oct 14, 2010 8:08 pm

I have one I built nearly 4 years ago. Last year the two 6BQ5 P-P output tubes died. Upon inspection I found I was running the tubes way over voltage as I was using a 115 Vac Hammond transformer on 120 Vac. Why all their transformers are not 120 Vac I'll never know. Maybe they use 115 Vac in Canada? I inserted a 5 Vac transformer to buck the 120 Vac to 115 Vac. That got the voltage closer, but there is supposed to be 300 Vdc on both 6BQ5 plates. I guess the modern chokes and audio output transformers have less dc resistance than the old ones did?Now given I only know the voltages to the 6BQ5 tubes and no other voltages do I reduce the voltage to just the output transformer or to the whole amp? I found that a 900 ohm resistor will get me to right below 300 Vdc. I pulled one from a defunct CRT computer monitor board, but it was giving off a little smoke so I am ordering a 10 watt resistor. In addition to dropping the voltage to where it is supposed to be it will also provide better filtering in the power supply.
Back to top Go down
View user profile
Johnnysan



Posts : 64
Join date : 2010-10-12

PostSubject: Re: GEC 912 PLUS amp   Fri Oct 15, 2010 12:38 am

You might consider replacing the power transformer. Maximum plate voltage for the 6BQ5 is 300; typical is 250. Using a resistor in series with the B+ means that the output voltage is going to fluctuate under varying loads, especially when reproducing bass notes.
Back to top Go down
View user profile
hergi

avatar

Posts : 2
Join date : 2010-10-13
Location : Fort Washington,MD

PostSubject: Re: GEC 912 PLUS amp   Fri Oct 15, 2010 11:30 am

Hi, if your power supply filter is C-L-C you might try removing the first filter cap making it a choke input. That will lower your voltage. Easy to try to see if works and cheaper than replaceing the power transformer. Just a thought.

Doug
Back to top Go down
View user profile
Tube Radio



Posts : 31
Join date : 2010-10-12

PostSubject: Re: GEC 912 PLUS amp   Fri Oct 15, 2010 12:40 pm

I tried and it didn't provide enough voltage. Now I could try to use something less than 10 uF for the first filter cap and see if that works. I'm thinking that the choke that would have been used originally when this amp circuit first came out may have had a higher dc resistance along with the output transformer. Also I used a 5U4 instead of the 6 volt rectifier tube specified so that may drop less voltage as well.
Back to top Go down
View user profile
Tube Radio



Posts : 31
Join date : 2010-10-12

PostSubject: Re: GEC 912 PLUS amp   Fri Oct 15, 2010 7:14 pm

I decided to try adding a choke in series with the rectifier and first filter cap. It just so happened that I tried the primary of an old power transformer and I get 300.5 Vdc on the 6BQ5 plates.

I just checked and I got 38 mA of cathode current now. I do believe the transformer I am using must be of a lower resistance than what was originally used as I should get 295 Vdc on the screen grids when I have 300 Vdc on the plates and for a voltage of 305.4 Vdc on the plates I get a screen grid voltage of 306.6 Vdc.

Aside from replacing one 270 ohm cathode resistor that nearly burnt up when one 6BQ5 went I am calling this amp done.
Back to top Go down
View user profile
tubes4life



Posts : 33
Join date : 2010-10-12
Location : northern Florida

PostSubject: Re: GEC 912 PLUS amp   Fri Oct 15, 2010 8:27 pm

Or, you could run 7189's, which have a more robust voltage rating...just a thought.
Back to top Go down
View user profile
Tube Radio



Posts : 31
Join date : 2010-10-12

PostSubject: Re: GEC 912 PLUS amp   Sat Oct 16, 2010 3:16 pm

I could, but since I already have the 6BQ5 tubes and got the voltage on the plates down to 300 Vdc There is no need.

Unfortunately I have no info on this amp in the book I got except what the operating conditions for the 6BQ5 tubes are. Apparently reducing the total voltage was a bad idea as the amp now distorts pretty easily and the distortion is coming from the phase splitter stage.
Back to top Go down
View user profile
dberman51



Posts : 19
Join date : 2010-10-13

PostSubject: Re: GEC 912 PLUS amp   Sun Oct 17, 2010 7:23 pm

Don't sweat the difference between 115 and 120VAC on the transformer primary. They are effectively the same.

300VDC is the design center rating for the 6BQ5, which means that it's OK to design to that value in normal operation even if it goes a bit higher sometimes. Other tubes are rated as design maximum, which means never ever exceed the rating.

What kind of B+ were you getting on the 6BQ5s before you started making changes? I doubt that was the reason the 6BQ5s burned up. More likely causes would be low/failed bias supply, or failed coupling capacitor driving grids positive.

-David
Back to top Go down
View user profile
Tube Radio



Posts : 31
Join date : 2010-10-12

PostSubject: Re: GEC 912 PLUS amp   Sun Oct 17, 2010 7:33 pm

I was seeing nearly 400 Vdc and was the reason for the red plating of the tubes as everything else in the amp is good. Reducing the input voltage to 115 Vac and adding another choke between the rectifier tube and first filter cap brought B+ down to 300 Vdc and the 6BQ5 tubes no longer red plate. I really need to find out what the voltages are throughout the amp though to make sure everything has it's proper operating voltage so the amp will for sure work properly.
Back to top Go down
View user profile
Sponsored content




PostSubject: Re: GEC 912 PLUS amp   

Back to top Go down
 
GEC 912 PLUS amp
View previous topic View next topic Back to top 
Page 1 of 1

Permissions in this forum:You cannot reply to topics in this forum
Antique Radios Forum :: Antique Radio Forums :: Electrical/Mechanical Restoration-
Jump to: