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 Voice Of Music amp problem

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Tube Radio



Posts : 31
Join date : 2010-10-12

PostSubject: Voice Of Music amp problem   Tue Oct 12, 2010 7:38 pm

I posted a similar topic about this amp on the forums and wanted to update on the progress I am making.

I had replaced the RCA jacks with 1/4" stereo jacks insulated from the chassis and used the star grounding method. Anyways I had a hum that was present. I inserted a thing I have for measuring the tube voltages above the chassis in between the 6BQ5 for the left channel and the tube socket and the hum went away. I then investigated and moved a couple grounds to another mounting tab on the filter cap can. That helped some, but when I removed the ground wire from the filter cap can that went to the tuner power jack (tuner not being used and is disconnected) the hum was reduced in level. I then probed the various grounds with that wire and found when that wire is connected to the ground terminal on the input jack the hum is gone completely.

I honestly have no clue why that fixed it as the wire is only grounded at one end and the connects to the tuner power jack.
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Don Cavey



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Join date : 2010-10-12
Age : 69
Location : Bltimore, MD USA

PostSubject: Re: Voice Of Music amp problem   Tue Oct 12, 2010 7:44 pm

Tube Radio wrote:
I posted a similar topic about this amp ...
I honestly have no clue why that fixed it as the wire is only grounded at one end and the connects to the tuner power jack.

HHmmmmm. Close to Halloween, maybe a ghost? No, seriously, thanks for the update...
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Tube Radio



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PostSubject: Re: Voice Of Music amp problem   Tue Oct 12, 2010 7:49 pm

You're welcome. Originally the grounds in the amp were connected in a series string and run near the chassis. I'm wondering when I did the star grounding if having the individual ground wires near the power transformer wires induced some hum in them and maybe the ground wire to the tuner power jack (tuner runs on 120 Vac with it's own internal power supply) picked up hum that was opposite in phase phase and canceled out any voltage on the ground wires?
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Tube Radio



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PostSubject: Re: Voice Of Music amp problem   Wed Oct 13, 2010 10:48 pm

I decided to check voltages on one of the 6BQ5 tubes and here's what I got
Plate 269 Vdc
G2 296 Vdc
Cathode 8.26 Vdc
Cathode curent 50 mA

I didn't actually measure the cathode current. I calculated it using ohms law by dividing the cathode voltage by the 164 ohm cathode resistor.

I got nearly identical voltages on the Magnavox stereo SEP 6BQ5 amp I have and AFAIK that amp ran 6BQ5 tubes ok. I redid the V-M power supply to match the Magnavox power supply which reduced power supply ripple. I think the 6BQ5 tubes were run a little hotter before the power suply mod as the output transformers were connected to the first filter cap after the rectifier.

Is it ok to have the screen grid running at a higher voltage than the plate?
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Don Cavey



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Location : Bltimore, MD USA

PostSubject: Re: Voice Of Music amp problem   Thu Oct 14, 2010 7:40 am

Screen voltage is often taken from the second filter capacitor so it is a few volts less than the plate which is usually taken from the first filter. And that cathode current for a 6BQ5 sounds high. Actually sounds like it is for both (PP) tubes?
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dberman51



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Join date : 2010-10-13

PostSubject: Re: Voice Of Music amp problem   Thu Oct 14, 2010 1:59 pm

I think 50 mA cathode current is fine for a 6BQ5. Remember this is Class A. Although I've never heard the term, my guess is that "SEP" is being used to indicate Single-Ended Pentode or something like that. So no push-pull here.

50 mA is the cathode current you see on the 50C5 output tubes in AA5 radios.

-David
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Tube Radio



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PostSubject: Re: Voice Of Music amp problem   Thu Oct 14, 2010 2:53 pm

Yes it is Single Ended Pentode.

I decided to take the voltage measurements directly from the tube sockets and got more realistic voltages. I did notice that when I inserted that adapter I has a high level of hiss in the left channel. Maybe there was some oscillation that was giving me weird voltages?
Here's the voltages I get. I had to take the measurements pretty quick as there are 5 homes on one pole mounted transformer and voltage does fluctuate at times.
Left channel

Plate 268.5 Vdc
Screen grid 272.8 Vdc
Cathode 8.25 Vdc
Cathode current 50 mA

Right channel

Plate 267.7 Vdc
Screen grid 272.8 Vdc
Cathode 8.32 Vdc
Cathode current 50.1 mA

I am not sure how much of the current is plate current and how much is screen grid current as I didn't want to break the circuit to insert an ammeter. I calculated cathode current based on the voltage across the 164 ohm cathode resistor that each tube has.

Judging by the tube data the tubes should be well within their limits. Now I got a matched quad of Tube Amp Doctor 6BQ5 tubes as I needed two for a mono amp I am gonna use to power a pair of Magnavox 16 ohm 12" speakers to boost the low end some.

Those Tube Amp Doctor 6BQ5 and 12AX7 tubes made a difference in the sound. The bottom end seems more full, but I will still need a little boost though from those 15" Magnavox woofers. I will definitely be going the powered route as the necessary choke if I connected them to the main speakers would be around 100 mH affraid
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dberman51



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PostSubject: Re: Voice Of Music amp problem   Thu Oct 14, 2010 7:18 pm

Tube Radio wrote:


I am not sure how much of the current is plate current and how much is screen grid current as I didn't want to break the circuit to insert an ammeter. I calculated cathode current based on the voltage across the 164 ohm cathode resistor that each tube has. Now I got a matched quad of Tube Amp Doctor 6BQ5 tubes as I needed two for a mono amp I am gonna use to power a pair of Magnavox 16 ohm 12" speakers to boost the low end some.
You probably have a couple of mA of screen grid current, but in a Class A amplifier the cathode current consists mostly of plate current.

If you're going for big bass you will probably want to do better than a couple of 6BQ5s.

-David
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Tube Radio



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PostSubject: Re: Voice Of Music amp problem   Thu Oct 14, 2010 7:55 pm

I don't necessarily want big bass. I just want to fill out the bottom end and the GEC 912+ amp I am gonna use worked decently, but I didn't get a chance to run the V-M amp up to high volumes with the GEC 912+ amp to see if 14 watts would be enough.

From the tube data I see where there can be sometimes around 11 mA of screen grid current.
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