Blew HT fuse on 800 clone

[Turd Ferguson]

I got a friends 800 clone and it blew the HT fuse so I figured it was a power tube. swapped out for a new set of both pre and power tubes, replaced fuse and when I powered it up did not blow fuse but as I went to bias the new tubes it made a sound like a motor boat thru the speaker well come to find out the new PI tube I put in fried!! I looked at the voltage chart for the 800 and on the PI V3 tube shows no voltage on pins 2 and 7 I am getting 24. volt here. i put a good tube in V3 and it plays now but it went from on of the best JCM 800 tone to almost unusable amp the gain is unusable above 2 and it sounds congested. what change? it blew that fuse the first time for some reason

please help

[Tazin]

The voltage present on pins 2 & 7 of V3 is normal (approximately 25vdc). The motor boating may have been just from the fact that the chassis was picking up stray frequencies since it was probably out of the cabinet. Does the amp have any grid stopper resistors (i.e. 5K6) installed on the output tube sockets? If not, then that could also cause the motor boating depending upon the tubes installed (new vs NOS).

[Turd Ferguson]

The motor boat sound was the PI V3 tube when I replaced it the sound stopped. yes there are grid stoppers on both power tubes. I just dont understand why it sounds so shitty now!!

thanks

[danman]

Have you tried another PI tube?

[Turd Ferguson]

Well I put a new tube in that I ordered and it was fried and then I put a tube in that I had known to be a good one other then that have not tried another one!

[Littlewyan]

Is it definitely biased properly? If the bias is too hot then you'll get that congested gain.

[Turd Ferguson]

Well the plate voltage was 490 volts and I bias it at 35mv

[Littlewyan]

Yeah thats spot on.

Are the EL34s redplating at all whilst you're playing?

Has the tone changed drastically? Might be worth trying other pre amp valves as you said you changed the pres as well as the power valves. V3 died on you, could be a bad batch. I've had it before where there was a short between the heater and cathode, but the only thing I noticed was a slight drop in gain and the amp would only 'sometimes' motorboat when turning it on. Took ages to figure that one out!

[Turd Ferguson]

Ok I tried all different per tubes and power tubes still the same!! the amps gain sounds farty

[Turd Ferguson]

I was thinking It blew the HT fuse so if a power tube shorted it could have fried a screen grid resistor or a output grid resistor.

what do you think?

[Littlewyan]

Farty? As in blocking distortion farty? Or cutting out farty?

Its possible it could have fried a screen resistor. Turn the amp off, drain the caps and measure it to see what you get.

Can you also measure your plate voltage, screen voltage, HT Voltage and the HT Voltage at the pre amp end. Just to see if anything funky is going on.

[Turd Ferguson]

Sorry for the delay been real busy!

this is the voltage readings i got

V1 p1ns
(1) 102V (3) 17 mv it should read 1.6 V (4) 3.2 AC (5) 3.2 AC (6) 196 V (8) 1.7 V (9) 3.2 AC

V2 pins

(1) 160 V (3) 1.029 V (4) 3.2 AC (5) 3.2 AC (6) 286 V (7) 160 V (8) 161 V (9) 3.2 AC

V3 pins

(1) 227 V (2) 22.8 V (3) 38.2V (4) 3.2 AC (5) 3.2 AC (6) 213 V (7) 23.7 V (8) 38.2 V (9) 3.2 AC

V4 pins

(3) 487 V (4) 483 V (5) -39 V

V5 pins

(3) 488 v (4) 484 V (5) -39 V



The power tubes V4 & V5 voltage are way higher then the chart I have tubes in and the amp is connected to the speaker

on V1 pre tube pin 3 getting very little voltage as you can see on the V1 readings above


can someone help

Thanks

[Haze13]

Cathode cap shorted? Explains the low voltage reading on pin 3 and the high voltage drop on the plate resistor.

[Turd Ferguson]

Okay I put new sozo .68uF cathode cap in still same results crappy gain!! I turned the lights off before I turned power on and V1 tube had a flash inside tube this is like the 3rd tube I have tried in this spot. checked voltage on V1 pin 3 after replacing cathode cap and still getting around 17mv instead of 1.6 V. as I said the amp was one of the best sounding 800 I have ever played and then it crackled and blew the HT fuse. so I replaced the fuse and a new set of tubes in biased it at 30mv to 35mv tried it on the high side and the low side still sounds like shit the 1K screen grid resistors and the 5.6k output grid resistors check within specs. still no progress

Thanks for all the help guys

maybe someone on this forum has had this problem don't know!!

[danman]

It may help to reflow all connections at the v1 socket, the connections at the board and cathode grounds. Is the socket good and tight and making a solid connection with the tube's pins?

If you plug into the low input, does it still sound bad?