Question about tube pins and schematics

[Rick1114]

I started replacing the filter & bypass caps on my 57' GA-5 last night. What a baby rats nest - looks nothing like my clean 66' Vibrochamp inside. I especially love how the filter caps are connected to piece of metal that is connected the chassis on one side only (of course it's one of these bolts you cannot remove or tighten) and swings from side to side. If your filter caps get bored they can swing around inside the amp!

Anyway, I printed some schematics & I noticed a some of the wiring did not match up as far the tube pins were concerned. For example, the bypass cap/resistor go to pin #8 of the 12AX7 in this schematic, but in my amp it goes to pin #3. What difference does this make?

http://www.informatik.uni-bremen.de/.../ga-5sl-jp.gif

Thanks,

Rick

[MWJB]

"the bypass cap/resistor go to pin #8 of the 12AX7 in this schematic, but in my amp it goes to pin #3. What difference does this make?" possibly none whatsoever. The bypass cap should be on the same triode that is fed by the wire from the inputs. E.G if wire from inputs goes to pin 2, then pin 3 has th bypass cap, BUT if wire from inputs goes to pin 7 then pin 8 gets the bypass cap.

Manufacturers often wired 12AX7 triodes backwards (input feeds triode B instead of triode A & vice versa), compared to schems & other amps.

[Robert M. Martinelli]

"twin" triodes

Hi,
probably in the real world it makes no difference, but twin triodes are not exactly "twin" as their name would suggest - due to the tube construction, the inter-electrode capacitances are not exactly the same, and one of the two triodes is more likely to pick up hum from the heater.

On older databooks (e.g. Philips ) it was clearly stated that the "most favorable" triode with respect to hum was the one connected to pins 6(anode),7(grid),8(cathode), so I think an early stage with a lot of gain would benefit from ensuring that the "better" triode is used there.

Newer datasheets ( e.g. JJ ) don' t even bother to mention this issue.

Anyway, I saw a lot of amps with "backwards wired" triodes, and never bothered reversing the triodes to see if it makes a difference, maybe I' ll do it next time I find such a thing and come back with the results ( if any ).

Again, probably it makes no difference, but now you know....

Hope this helps

Best regards

Bob

[Rick1114]

Thanks for the replies. I get the jist of what you guys are saying. I'll have to purchase some more books to learn exactly how the triodes work one day, but for now I'll just leave it be. The amp does not hum excessively, at least not when it worked it didn't!

Checked all the resistors last night, and they seem to be fine except the one resistor attached to the bypass cap going to the 6V6 (cathode resistor?) was showing 580 ohms, when it is supposed to be around 470. Is that far out of spec and could that contributed to my PT blowing up?

[MWJB]

As cathode resistor value rises, plate current drops, so the OT wouldn't have been having a particularly hard life. Replace the resistor with a new one (5W rating), the original may be drifting all overthe place when hot. Replace the bypass cap with a 50v min item.

OT probably just went south after years of hard work.

[Rick1114]

As cathode resistor value rises, plate current drops, so the OT wouldn't have been having a particularly hard life. Replace the resistor with a new one (5W rating), the original may be drifting all overthe place when hot. Replace the bypass cap with a 50v min item.

OT probably just went south after years of hard work.

I've replaced the bypass caps with 25uf/50V spragues last night. Put in all new filter caps as well. I tested all the resistors that had exposed leads, and they are all looking okay. Still no clues as to why my Power Tranny blew up & I am waiting on the new one.

Only issues I see physically is the tubes sockets were very loose, and there is a decent amount of rust on the internal solder joints and on the Power tranny as well.