[pabbi1]

I'm getting a sporadic 'hissing' on the left channel only, with and without input, which started a few days ago, along with an occasional brief 'scratch / buzz'. No doubt (?) this is a tube, but which one is most likely, input or output?

Input: VT231
Output: Cetron 7236
Rectifier: 5u4

Any suggestions are welcome, as I don't typically tube roll, and would prefer a starting point. The issue isn't replacements since I've got multiples of each, but rather the investment in burn in time.

Thanks in advance.

 

[studeb]

Quote:

Originally Posted by pabbi1 I'm getting a sporadic 'hissing' on the left channel only, with and without input, which started a few days ago, along with an occasional brief 'scratch / buzz'. No doubt (?) this is a tube, but which one is most likely, input or output? Input: VT231 Output: Cetron 7236 Rectifier: 5u4 Any suggestions are welcome, as I don't typically tube roll, and would prefer a starting point. The issue isn't replacements since I've got multiples of each, but rather the investment in burn in time. Thanks in advance.

do you get this on start up? or when its warm?
i'd check the caps and resistors before running down tubes. Easier unless you have a tube tester handy. Why don't you call around and see if someone near you has a tube tester. Wait, if you have multiple tubes swap them out one at a time. Find the bad tube and bury it.

 

[pabbi1]

Definitely when warm, after several hours of use.


I'm really hoping for some HA-2 owners who might have experienced this before, and have input as to which tube is _most_ likely the culprit.

 

[rickcr42]

just so you know the full owners manuals including the schemo with operating voltages etc is at Pete's website

The "tssst" is usually a sign of a tube going south on you and if there are no visuals clues such as one tube glowing red or having a gassy appearance it comes down to process of elimination.

one channels says it is not the rectifier tube or BOTH channels would have a problem so it is gain stage or output section.

swap the left and right output tubes and if the problem moves to the other channel it is the tube in the bad channel,if the problem remains as is it is the gain stage tube

 

[rickcr42]

http://www.pmillett.com/wheatfield.htm

 

[studeb]

Quote:

Originally Posted by rickcr42 just so you know the full owners manuals including the schemo with operating voltages etc is at Pete's website The "tssst" is usually a sign of a tube going south on you and if there are no visuals clues such as one tube glowing red or having a gassy appearance it comes down to process of elimination. one channels says it is not the rectifier tube or BOTH channels would have a problem so it is gain stage or output section. swap the left and right output tubes and if the problem moves to the other channel it is the tube in the bad channel,if the problem remains as is it is the gain stage tube

The tubes are both dual triode so both channels are in the same tube. He has spare tubes, i suggested he swap them one at a time to identify the bad tube.

 

[rickcr42]

checking the schemo right now......wait five......

 

[rickcr42]

OK


simple gain stage to CF so again just do a interchannel tube swap.if the problem moves to the other channel it is that tube needing to be replaced

 

[studeb]

Quote:

Originally Posted by rickcr42 OK simple gain stage to CF so again just do a interchannel tube swap.if the problem moves to the other channel it is that tube needing to be replaced

Are you looking at the HA-1 or HA-2???????
i'll give you another five.......

 

[rickcr42]

http://www.pmillett.com/Wheatfield/manual.pdf

 

[rickcr42]

anything ?

 

[studeb]

Sorry i have other fish frying at the moment

If you look at the schematics, there are three tubes.
V1 - 5U4 which is the rectifier
V2 - 6080 (6AS7G) as a cathode follower
V3 - 6SN7 for voltage amp

V2 and V3 are dual tubes, each half handles one channel. That is why there is no heater for the lower 6SN7 V3-2 or upper 6AS7G V2-2

 

[rickcr42]

guess maybe I should look at it but problem is,I can post but not DIVERT at the moment unless I want to get fried with 350 volts ! Won't do to look at two schmos in the same time frame at my age and attention span level

just stepping back from the bench every few minutes to read email and respond to threads,will get back to you in a few,promise

 

[rickcr42]

Just used the "+" key in acrobat reader and oops !

makes it a bit rougher to do a channel swap when one section of a dual tube is used for each

anywa...............

chances are if it is a tube going bad it would be an output section tube or the rectifier with the only real way to know is either test the tube or swap in a known good one to be sure.There really is not much else that can go wrong with such a design and tubes run hot (se class-a cathode follower,powr supply for two channels) just wear out over time and become noisy before they totally go silent.

would be an ideal time for some good old "tube rolling" experiments

 

[pabbi1]

Thanks for the links - I had those handy. It turns out (apparently) to be the 6sn7 (Vt231). I just couldn't face the possibility of it being the 7236...