Emergency case: Pulling a pair of tubes

  • Thread starter Thread starter petereanima
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petereanima

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so, seems like 1 pair of power-tubes just died (tube-fuse-led went on). i will get some spare-fuses today and check again if maybe only one of the tubes didnt sit correct in the socket after transport or if the fuse blows again.

but i need the amp tomorrow evening, so just in the emergency case: if i pull tube 1+6, how does the impendance change (cab = 8 ohm mono, 16 ohm stereo)? am i right that i have to connect the 16ohm output of Herbi to the 8ohm input of the cab then?

thank you in advance!
 
Leave everything as it is, just pull them out. Little impedance mismatch wont hurt anybody.
 
steppenwolf3080":193m9xfa said:
Leave everything as it is, just pull them out. Little impedance mismatch wont hurt anybody.
Click to expand...
+1
 
When you say "pull 1and 6" I take it you are talking about a Herbert?
 
yes, i was talking about Herbert!

duesentrieb":310xiqyk said:
steppenwolf3080":310xiqyk said:
Leave everything as it is, just pull them out. Little impedance mismatch wont hurt anybody.
Click to expand...
+1
Click to expand...

thank you guys!!
 
seems like it was just the fuse, i replaced it and everything went fine, was playing about 3 hours (LOUD!) and nothing happened. :)

i will nevertheless by new tubes soon, and this Bias-measuring-adapter from TT...i'm sure i can do this by myself (and the next tech is over 1h drive away :aww: ).
 
You can buy it presoldered. All you additionally need is a DMM from a Baumarkt. ;)
 
yeah, i already have a DMM, but i will buy a new one which is save to 600V, just to go for sure. :)

thank you again!
 
It's of course no waste of money buying a good quality dmm but when measuring your bias current with the TT adapter you don't actually measure current but voltage drop across a 1Ohm resistor. So in that scenario you can expect voltages of something around 100mV, thats 0,1V.
You should be on the safe side ;)
 
P.A. may want to read the exact plate voltage as well . . . ;)
 
Right! But you must pull out the chassis or at least remove the tubes and measure from outside the socket. Then, of course, you will have a higher reading because of the smaller current flowing in the PA section. Using this plate voltage to set the bias can give you a lower bias point when calculating with this voltage. Then again, BIAS setting is not an exact science especially in tube amps ;)
I just wouldn't recommend pulling the amp out of it's case without proper technical knowledge and keeping away from high voltages in general. It can get pretty dangerous.
I think you can expect something around +480 - +490 Volts. It should be sufficient to calculate with that value.
Do you have the external bias adjustment pots? If not, you will have to get the amp out of the head in any case...
Best,
Stefan
 
Stefan, with the (DIY) TT adaptor you can measure the plate with all tubes (and the adaptor) installed from the outside (still the bias pot is in the inside of older Herberts) - as long as you know where to read it :D ;)
 
Really? I couldn't see a third wire coming out of those adapters. Correct me if I'm wrong but to measure the B+ with the adaptor, you'd have to connect a third wire and run it out to your dmm and all that with 500V to ground. That would come just before suicide. Where else would you measure it?
 
i bought the amp new in february (serial no. 001/981) so i guess about 11-12 month. they are TADs.
 
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