Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: genpk on May 14, 2021, 07:19:59 am
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Hi all, new to this forum.
Found an old Tiesco 73 amp in half working condition.
Has 2x 6bq5 output tubes, 2 x 12ax7 and 2 x 12au7 pre amp tubes, tremelo and echo effects.
Been replacing a few electrolytic caps and came across some some old oil filled electrolytic
caps marked with .1 (M) 400v
My question,is .1 the same value
as 1uf ? or is it .1 of 1 uf.
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.1 is .1uF
1 is 1uF
1uF is ~~ where electrolytic caps start, that not a "rule" just a notation to verify which it is
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thanks for the info, interestingly noticed a big 75ohm 20 watt resistor on the mains power feed to the transformer was heating up alot, not being an amp guru, have replaced the electrolytic caps and filter caps which seems to have solved that issue but i not seem to have upset the bias
on one of the output tubes which is red plating. Cant find any schematics for this amp, im guessing its similar to tiesco checkmate 20 with the voltage doubler, but has a tremelo and
echo as well.
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20W resistors get hot. it's too early to hunt schematics, with all the tubes out it shouldn't get hot typically. If it does, there might be leakage somewhere
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ive changed the filter caps and all the old oil filled electrolytic caps which seems to have stopped the
20w resistor heating up, tremelo has started to work, down side
is one of the 6bq5 output tubes
is red plating and as such a very gravely sound.
Checked all the resistors and all appear good. Have just replaced
all the old caps on the board so will see how we go tomorrow.
Given that the mice had set up residence in the amp and chewed
up bits of the wiring i did some online detective work and found a couple of pictures of the tiesco 73
which showed the electronics but non appear to have the echo effect in them.
The only wire i couldnt place was the signal wire from the echo footswitch which seems to attach to the negative end of an 30uf 15v electrolytic .
Im familiar with A/B amps but
this japanese amp is another whole thing !!
Im learning - bit by bit !!!
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is one of the 6bq5 output tubes
is red plating and as such a very gravely sound.
If bias resistor is good
1- tube socket issue ( no cathode bias )
2- Tube is short
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yep, only one 6bq5 is red plating.
only happened after replacing the electro caps on the board.
I give it another go today as ive replaced a few of the old mica caps on the board.
As im in Australia, we have 240 volt mains power, notice that 20 watt resistor drops the voltage down to around 200 volts to the transformer.
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Reverse both 6BQ5.
if it's the same redplating; this tube is short
If is is the other one, something is wrong with the socket or wires at the socket
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been tinkering again today, still red pkating on the same tube.
Found one dead grid resistor thinking it was it changed it but still the same tube red plating on that same tube, swapped to another tube, still the same.
I notice the tremelo volume seems to be acting like a volume control as well.
Another interesting thing, the HT
from the caps seems to supplying
360-380 volts to the 6bq5 oitput tubes, is that a normal voltage for these tubes ?
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Another interesting thing, the HT
from the caps seems to supplying
360-380 volts to the 6bq5 oitput tubes, is that a normal voltage for these tubes ?
Yes, 400 or less is good
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its interesting as when i switch the amp on for the 30 seconds it sounds awesome until the distortion starts kicking in.
I have just about replaced everything on the boards except
a couple of .001 mica caps on one of the boards and some resistors.
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When bias is a way too high ( tube red plating ) , amp always sound very good....... for short time.