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Re: Help.....I can't stop the buzzzzzzzzz!

11/14/1998 11:48 PM
Re: Help.....I can't stop the buzzzzzzzzz!
I found in my last creation, initially I didn't use any sheilded wire. I never had a problem till this little boy. I use some shielded wire on the input, from the volume to the second stage. But where I got the most was from the coupling cap from the second stage to the driver. I was about a 2" run but knocked the noise down about 60%.  
Hope ya get it worked out  
11/15/1998 8:10 AM
Benjamin Fargen

Brent, Thanks for the input......I pulled the second stage tube(6SH7) and the noise was a reasonable level. I think I might try some shielded cable as you suggested. If you look at Duncans tube data section on the 6SH7, it says that this tube is a poor choice for audio. I wonder if this is why Matchless changed it to a EF86(?)All I know is that this is one of my favorite sounding amps so far.......and of course I have this noise issue! Hopefully I can figure it out in a reasonable amount of time!  
Thanks again,  
11/15/1998 9:23 AM
Benjamin Fargen
Re: Tube data notes on the 6SH7......any thoughts?
It reads: "This type of tube is not recommended for high-gain audio applications because an undesirable hum may be encountered in circuits where the cathode is not directly connected to the heater(the Clubman circuit does not do this),the potential difference between heater and cathode should be kept as low as possible"  
Is there any way to modify the circuit to accomodate these requirements for lower noise?  
How do I check the potential difference between the heaters and the cathode?  
Thanks, I think this is an interesting problem.  
11/15/1998 8:42 PM

Benjamin: Am not familiar with the amp in question and have just now read down the thread. Where is the 6SH7 in circuit? Assuming a guitar level input for the amp of 0.3 - 0.5v , what is the signal input to the gird of the tube and what is the output? I ask this to determine what level of hum source we're looking for. Oh, is this a 60Hz or 120Hz hum? Have you tried several 6SH7s to see if some tubes are better than others?  
Checked my '53 RCA Tube Manual and found verification of the potential hum problem:  
"Metal type used as rf amplifier in high-frequency, wide-band applications .. similar electrically to miniature type 6AU6...two separate cathode terminals enable the input and output circuits to be isolated effectively from each other. This typs is not recommended for high-gain audio-amplifier applications because undesirable hum may be encountered."  
So, if this tube is in a high-gain stage and you can't find one that seems to hum less I'd consider constructing an adapter from 6SH7 to 6AU6, plug one in, and see if the hum diminishes/goes away. If not, it's probably not the 6SH7, if so, you might see how much you like the 6AU6's tone - many were used in audio applications.  
11/15/1998 8:56 PM

Follow-up post. Depending on how the socket is wired you could probably temporarily substitute a 6SJ7 instead of crafting a 6AU6 adapter. The 6SH7 uses both pins 3 & 6 for the cathode with the supressor grid internally connected to the cathode. The 6SJ7 uses pin 6 for the cathode and pin 3 for the supressor grid - not internally connected to the cathode. So, you'd need to jumber pins 6 and 3 to try the 6SJ7.  
The 6SJ7 characteristics are somewhat different but close enough to determine where the hum problem source is.  
11/16/1998 7:42 AM
Benjamin Fargen
Re: 6SH7 conversion to 6SJ7?
Graywater, Thanks the great all I have to do is jump a lead across 6 and 3 when using the 6SJ7 and the rest of the connections will still be correct?  
Thanks for the help, Benjamin.
11/16/1998 8:05 AM

Yep, but... the 6SJ7 characteristics are different enough that the tube might not have the same sonic characteristics and might distort more easily - it actually "wants" a higher control grid bias. My suggestion is, first, to determine if the problem is the 6SH7 or something other and, second, to suggest an alternative tube for the circuit with possible component modifications.  
Lotsa luck GW

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