Screen voltage - standby cut (Roland Bolt 60) - diyAudio
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Old 11th March 2005, 09:12 PM   #1
jjai is offline jjai  United States
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Default Screen voltage - standby cut (Roland Bolt 60)

I just purchased a Roland Bolt 60 - a 60W guitar amp circa 1980 that has a SS preamp and tube power amp (2 6L6s and 1 12AT7 phase inverter).

I've been doing some research and discovered that there is apparently a design flaw -- the standby switch only cuts plate voltages, supposedly causing the screen current to exceed the "limits allowed by tube manufacturers."

I'm a bit of a tube newbie, and I'm wondering - is this something worth fixing? Will this problem damage the amp/make me run through tubes like crazy?

I don't have a schematic, and I'm sort of a newbie, so I'm a little wary of tearing into my the amp and tinkering (especially after unsucessfully trying to fix another amp for 2 months before giving up).

Any suggestions/advice/copies of the schematic for this amp would be much appreciated!

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Old 12th March 2005, 12:12 PM   #2
Giaime is offline Giaime  Italy
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First of all for all tube newbies: if you aren't qualified to work on extreme dangerous high voltage tube equipment, just don't do it: refer to a qualified technician.

For the amp problem: it is possible that the amp (like many) was designed with the strong tubes of the past ages. Many new production tubes aren't strong enought to survive to those things... for example an RCA could do it but not a cheap Sovtek...
I think that to solve the problem you could:
- add some screens resistors to limit current on the screens, 1k 5w for each tube will do;
- move the standby switch to the center-tap of the B+ winding on the power transformer. If the original switch was a SPDT it is the best solution, if it is a DPDT you can switch both the B+ wires from the PT.

Remember that this is dangerous work...
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Old 23rd May 2005, 06:26 AM   #3
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Default solving Bolt 60 standby switch design mistake

before doing anything with your bolt go to:

there you have the info about the standby switch modification, and you have also the schematics , manual and service notes for the bolt 60, don't waste your time with the message above, go only on this site.
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Old 23rd May 2005, 11:33 AM   #4
Giaime is offline Giaime  Italy
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Default Re: solving Bolt 60 standby switch design mistake

Originally posted by rolandbolt
don't waste your time with the message above
Thank you very much!
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Old 23rd May 2005, 11:50 AM   #5
SHiFTY is offline SHiFTY  New Zealand
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Don't use the standby switch
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