VOX AC50CPH - cant bias one tube ?! (no value read)

Solutions, Advice, Mods, and Troubleshooting.

Moderator: longfxukxnhair


HF1600ie
<200 Posts
<200 Posts
Posts: 188
Joined: 25 Jun 2009, 07:27
Has Liked: 2 times
Been liked: 20 times
Contact:

VOX AC50CPH - cant bias one tube ?! (no value read)

Postby HF1600ie » 18 Sep 2016, 05:23

Hi,
Well, I just acquired a Vox AC50CPH. All was working, checked bias for EL 34 which was close to 38mv on each valve. Fine tuned it and enjoyed the amp for a day.
Then, I decided to try my Marshall VM2266 KT66 output valves in there. Placed them in, checked for bias and for as much as I wanted, I couldn´t reduce them below 60mv, so as I coudln´t properly adjust them, I removed them. (Pitty, because they sounded great....)

When I was going to put back my EL34, I didn´t notice that one of them had the base plastic "guide" broken. So, as I felt it was going down, I just inserted it.
When I turned on the amp, I sudeenly heard some "hum" (half a second), and then.... silence. The "green light" from the standby switch didn´t light.
HT Fuse blew.

I replaced the HT fuse with another of same reference (from the Marshall 2266), then, properly inserted the EL34 valve and the amp worked again. BUT...

- Both EL34 light up.
- I can hear my guitar.
- I am only able to adjust the left one to 38mv. The multimeter doesn´t erad anything at all in the right valve.
- I tried an other EL34 I had spare, even swapped their places, etc, etc. Both EL34 still light up, but I can never adjust the right valve bias.

Also, the multimeter doesn´t detect any resistance in the right potentiometer, reading from the probes. But It does for the left one.

What could this be ? Can anyone give me a hint ? If too complicated, I´ll send it to a tech.

Thanks !

Anitoli
Knighted
Knighted
User avatar
Posts: 4532
Joined: 05 May 2013, 12:35
Location: Lewiston, Maine
Has Liked: 2610 times
Been liked: 2688 times
Contact:

Re: VOX AC50CPH - cant bias one tube ?! (no value read)

Postby Anitoli » 18 Sep 2016, 06:39

First off, KT66's are not drop in replacements for EL34's. The screen grid resistors are of a different value as well as other parts. The reason they sounded so good is they were prolly on the verge of a melt down as the bias voltage for EL34 is much lower than what is needed for KT66.

Now, when that EL34 was inserted in the wrong position, you more than likely have some burned components in there due to the mis- alignment of the pins.

At this point i would strongly recommend taking it to a tech.

And finally, don't just throw any tubes in there, they are NOT all electrically the same. Use what the amp is designed for.

HF1600ie
<200 Posts
<200 Posts
Posts: 188
Joined: 25 Jun 2009, 07:27
Has Liked: 2 times
Been liked: 20 times
Contact:

Re: VOX AC50CPH - cant bias one tube ?! (no value read)

Postby HF1600ie » 18 Sep 2016, 06:41

By the way, just to add that if I adjust the bias on the left probe higher, I clearly hear an immediate effect of overbiasing which increases the hum.

But on the left side, there´s absolutly no effect at all, no matter where the pot position is. And it reads zero regardless of the position.

HF1600ie
<200 Posts
<200 Posts
Posts: 188
Joined: 25 Jun 2009, 07:27
Has Liked: 2 times
Been liked: 20 times
Contact:

Re: VOX AC50CPH - cant bias one tube ?! (no value read)

Postby HF1600ie » 18 Sep 2016, 06:45

Hi Anitoli.
I´ve read somewhere (still my fault) that KT66 would have the same pinout as the EL34. So, I gave them a try.

Anyway, The issue really happened after the wrongly inserted EL34.

At this moment, the amp works, I can play guitar, both valves light up and get warm, but for only one of them I can set bias. For the other has absolutly no reading on the probe.
Yesterday I had a look inside and saw nothing looking bad. I know it doesn´t need to look burnt to be burnt....

I was trying to avoid taking it to a tech. The only one I know is very far and during working week there´s just no time !

HF1600ie
<200 Posts
<200 Posts
Posts: 188
Joined: 25 Jun 2009, 07:27
Has Liked: 2 times
Been liked: 20 times
Contact:

Re: VOX AC50CPH - cant bias one tube ?! (no value read)

Postby HF1600ie » 18 Sep 2016, 13:55

Ok, I had a look inside again and saw nothing toasted. Nothing smells either.
With the amp OFF, I used my multimeter and tested a few resistors.
R104, R109 R127 e R120 are good. R115 doesnt give a reading. It should be 1K 5W. The other one right next to it is fine.
R115 is a white square big resistor.

The two blue resistors directly attached to the poweramp tube sockets give a reading close to 5K each. I think they are R193 and R194. Are these the screen grid resistors ?

Maybe this is just about R115...
Attachments
vox_ac100cph.pdf
(4.16 MiB) Downloaded 175 times

Anitoli
Knighted
Knighted
User avatar
Posts: 4532
Joined: 05 May 2013, 12:35
Location: Lewiston, Maine
Has Liked: 2610 times
Been liked: 2688 times
Contact:

Re: VOX AC50CPH - cant bias one tube ?! (no value read)

Postby Anitoli » 19 Sep 2016, 02:01

R115 would be the screen grid. If this resistor is open it would affect the tubes operation. If the tube's heater elements are lighting up then the tube will get warm but not necessarily conduct current. Most likely that resistor was burned when the tube was inserted in the wrong position. You will need to change that.

HF1600ie
<200 Posts
<200 Posts
Posts: 188
Joined: 25 Jun 2009, 07:27
Has Liked: 2 times
Been liked: 20 times
Contact:

Re: VOX AC50CPH - cant bias one tube ?! (no value read)

Postby HF1600ie » 19 Sep 2016, 02:28

Hi,
Yes, the tubes light up. Both of them. In fact, any tube will light up and get warm.

I will take the amp to a tech today and also buy a 1K 5W resistor to take with me. From all the resistors i´ve tested, R115 was the one with no reading.
But there are a few others I should have tested as well, like R108 and R118.

I was thinking that maybe those which are connected to the poweramp tube sockets (one per socket) could be affected, but they gave readings.

Anyway, this shouldn´t be a great issue, I guess. But to make sure all is fine, I´ll take it to a tech.

HF1600ie
<200 Posts
<200 Posts
Posts: 188
Joined: 25 Jun 2009, 07:27
Has Liked: 2 times
Been liked: 20 times
Contact:

Re: VOX AC50CPH - cant bias one tube ?! (no value read)

Postby HF1600ie » 24 Sep 2016, 01:28

Hi everyone,
Jus to close the topic. I took the amp to a tech and he confirmed what I´ve already seen, R115 was toasted (Despite looking normal).
He decided to swap both cement resistor R115 and the one side by side to it as well.
Offered me a fuse to put back into the Marshall 2266, as VOX was using the Marshall´s.

Now bias can be tuned again. And is working perfectly.

Simple thing, all in all, just a fuse and a resistor were toasted but I think I made the right decision as other things could be envolved as well.
Cheers !

Return to “Roadhouse Repair Shop”

Who is online

Users browsing this forum: No registered users and 4 guests

cron