JTM45 Build with various output tubes and GZ34/Solid State Rectifier

[Spencerleehorton]

yep as i thought, had to be connected, now getting 465v on B+ wth diodes, will leave on now and see how hot the mains txf will get.
started at 23C its now gone up to 27C after about 5 minutes.
first 32uf cap connection is at 38C?
could it be a dodgy 32uf cap?

 

[Spencerleehorton]

ive used a bias calculator which was provided in this thread, and it said that for EL34 at 70% it should be 43mA with 403v which is what i was getting.
is the bias calculator wrong?

Im now at 443v B+ so will adjust bias to 39.5mA

BTW. have ordered some KT66.

 

[Spencerleehorton]

its been about 30 minutes now and its only at 31C, warm to the touch but doesnt seem to be going over 40C like before, the bottom end with diodes is much tighter as well.
will keep monitoring.

 

[Spencerleehorton]

45 minute mark and now gone up to 38C and still climbing!! getting pretty warm now!

 

[Spencerleehorton]

ive spoken with Michael at modulus amplification where i got the Mains transformer and Choke from and he has said that 39C is fine and not a problem.

 

[StarTrucker]

are you having any other problems with the amp?

 

[Spencerleehorton]

One of my concerns was it was getting rather hot, my only other concern is the bias, i am using a 68k for the 180k into the diode and a 5.6k then a 27k with a 50k pot, should i be concerned that this is vastly different than the regular setup of 180k into diode then 15k, then 56k into 50k trimmer?

 

[Spencerleehorton]

so a matched pair of KT66 have arrived to day and i thought, right , i'll test the voltage on CT which was 433v and use the tube calculator for bias.
i could see that both EL34 and KT66 needed 40mA for 70% A/B.
so basically i had it on 39mA with the EL34s and would just need to increase 1mA once i'd put the KT66 in.
here is where it gets weird, so i the previous post i put that my bias is now 68k into diode into 5.6k into 27k into 50k trim pot and im getting 39mA using EL34s.
Ive now put in KT66 matched pair and im getting 170mA and can only lower to 70mA?
so i thought ok well i'll change the bias resistors for the more regular values used on the original schematic, but first i'll change the 68k to 150k, now bias at lowest point is 180mA so thats gone completely the wrong way?
i want to understand whats going on before i continue?

 

[merlin]

Do you understand a potential divider? In a potential divider, when you make the series resistor bigger, you are making the output voltage smaller. It's like turning a volume pot down; you get less voltage out of it (i.e. the output gets closer to 0V). With a smaller bias voltage going to the tube grids, they bias hotter.

I admire your ambition, but you seem to be out of your depth with the level of complexity you're attempting.

 

[Spencerleehorton]

yep well out of my depth!!! but im learning, looking up about it now and will work it out.

 

[Spencerleehorton]

ok, i understand whats happening now and how to get what i want, Vs (source volts) x R2 (27k) divided by R1 (5.6k + R2 (27k) = V out in this case 41.4ma

source volts = 50v

 

[merlin]

= V out in this case 41.4ma

Volts are not amps...

Step 1: Take the damn tubes out.
2: Get the bias supply working so the bias voltage is adjustable from, let's say -10V down to -45V or so. That's enouge range to cover any tube type.
3: Put the tubes back in and bias them.
4: Stop pratting about with the sodding bias supply from then onwards.

 

[Spencerleehorton]

I have installed 150k into diode (1n4007) into 15k then 56k (voltage divider) then into 50k trimmer to gnd
at 150k / diode i get 115v
at diode / 15k i get -55v
at voltage divider i get 43.3v
when i measure the mA on the 1R resistors its at 180mA? not 40mA?

 

[Spencerleehorton]

ive changed the 150k now to 56k and im getting -43v max on bias, i dont know whats happened to the 1R readings on pin1/8 to gnd?

Ah i see my problem i think, im tapping off between the 56k and the 50k trimmer as a voltage divider rather than from between the 15k and the 56k!!!

 

[Spencerleehorton]

well its much closer now, ive put back the 150k into the diode and then have 15k and 56k with 50k trim down to gnd, im now tapping off the 15k/56k voltage divider!!! and getting -43.4v but when i measure mA on the 1R resistor on pin 1/8 to gnd its measure 68mv (this is meant to reflect the same as the bias but read as mA)

 

[merlin]

Text descriptions are hard to follow, so maybe I misunderstand you, but just in case:
You are aware the the bias current in the tube is not numerically the same as the bias voltage on the grid, right?
If you're aiming for 41mA in the tube (41mV measured across 1 ohm), do not expect the bias voltage to be -41V. The grid voltage and the bias current are two completely different numbers. You simply adjust the voltage to whatever it needs to be to achieve 41mA in the tube .
I don't mean to patronise, just better to clear this up.

 

[Spencerleehorton]

thats exactly what im looking for, im confused of what readings to follow and you have just said that the mv reading on pin1/8 is what i need to read and not worry about the -43v on the bias.
ive lowered the 150k to 133k but i need to go a little lower so ill try a 100k in there and it should be about right hopefully!!!

then i can leave you all alone for a while!!!