A pour broken and unloved AC4TV followed me home.

[NO_H2O]

I took the AC15 clone to my local music store to let the owner play it. As I was leaving, he ask if I wanted this broke AC4 so I hauled it home. It is missing the EL84, and some hardware. I plugged in an EH EL84 and no sound. Pulled the tubes and checked for voltage. It has good voltage at the right pins. While I had the fluke out I checked the speaker and it rings out ok. But the OT secondary shows open. But this thing has an output selector between the OT and speaker so it could be open there. I need to isolate it and recheck. If the OT is bad I will have to sort out a replacement. I was thinking that if the selector is bad rather that repair it, I think I would rather do away with it. WTF, it is a 4 watt amp. How often does one need to go to 1 watt or 1/4 watt? And if I have to replace the OT I will not want the selector mucking things up on the new one.
What say you guys?
Now let me see if I can get the schematic linked.

[kagliostro]

Now let me see if I can get the schematic linked

Which one of these two it is ?

http://el34world.com/charts/Schematics/files/Vox/Vox_AC4TV.pdf


http://el34world.com/charts/Schematics/files/Vox/Vox_ac4.pdf


Franco

[NO_H2O]

It is the first one. PCB with Attenuatore network. It is that network I need to isolate from the OT to see witch is open.

[NO_H2O]

Thanks for posting the schematics. I have it here on my iPad in iBooks but have not sorted a way to link it to post.

[NO_H2O]

I need to see if I can dig up a pair of computer speaker to mahe a listening amp. Anyone have a link showing a good method to fab the probes?

[kagliostro]

Are you looking for an Audio Probe ?






http://billmaudio.com/wp/?page_id=1254


https://www.youtube.com/watch?feature=player_embedded&v=IEcyDzYAsuU


Franco

[NO_H2O]

That looks to be it, thank you.
I de-soldered to OT from the Attenuator card and the secondary winding look to be good. The cheap rotary switch is all jacked up so I soldered the OT directly to the output. I now have sound. But the input jack could use some attention and my EH EL84 is not good (a bit of arcing and popping and on my tube tester it also test bad for grid leakage. My son has my stash of tubes at his house so I need to pick those up and see if I have a good one to try.

[NO_H2O]

Drove down to my sons place and grabbed some of my tube stash and my 81 Gibson 335S. I do have a single 6BQ5 from Great Britain.
I think this is a good Mullard. Can anyone confirm this tube?

[NO_H2O]

I plugged in the British 6BQ5 and a Phillips 12AX7 and the little Vox sure hand some spank to it. But I can't bring myself to keep them in there. So a JJ 12AX7 and an EH EL84 are in it now. It has a bit of a sizzle sound as the notes tapper off now. I think I will touch my iron to all the points on the board and pick up a JJ EL84. What to do with the empty hole were the opt level switch use to live?

[kagliostro]

What to do with the empty hole were the opt level switch use to live?

Rebuild the attenuator ?

SAG resistor on G2 ?

NFB Pot ?

RAW Control ?

MV pot ?

---

As power tube give a try to a 6P14P-EV

Franco

[NO_H2O]

I was thinking a cut control between C18 and R20. And add a cap across the Vol pot to brighten things up a bit to be controlled by by the cut control. Or am I off a bit.

[HotBluePlates]

... I do have a single 6BQ5 from Great Britain.
I think this is a good Mullard. Can anyone confirm this tube?

It sure looks like one.

Do you see where the "GT Britain" is etched into the glass at the bottom (your 1st photo)?  Look toward the left side of that etching, and there is a set of codes in 2 rows.  Post here what they say (they'll indicate the factory and date of origin).

[kagliostro]

I was thinking a cut control between C18 and R20

A Tone Cut control like in AC30 or you mean MV (Master Volume) ?

Franco

[NO_H2O]

I had thought about a tone cut. But I had been drinking. Maybe just a bright switch.
The code to the left on that tube looks like RMI or KM1 then 8283.

[kagliostro]

Someone that has practice with this thing can tell you at what correspond exactly those data

menawile if you want to make a bit of brain exercise

http://www.audiotubes.com/mullcode.htm

Franco

[NO_H2O]

kM1 B2B3 is my best on the code. It a bit hard to make out.

[kagliostro]

I think more to an initial R instead of K (R seems to be Mullard)

however take in consideration that also uppercase or lowercase is important, so report here the EXACT label write as is in the tube (as you read it of course)

an expert will be able to interpret it

Franco

[NO_H2O]

I have played around with this little amp. Did some reading online. I have swapped C5 to a .022uf cap and swapped C4 to a 500 pf cap. Next I may fill the now empty control hole (Attenuator circuit now gone) for a MV control pot between C18 and R20/C20. Not sure what pot value would be fitting. Any advice?

[NO_H2O]

The little amp sound much better. It went into brake up too early to me so I dug out a JAN 12AT7 and that sounds much better. I also got a RI "Mullard" EL84 to stab in and while not as punchy as the real Mullard, it does work well. I may look for a better 10 inch speaker and use some left over birch plywood and put it all in a better cabinet. I have less than $20 in it now so it's not like I will be out anything but a few hours of time.

[NO_H2O]

Well a couple mods, and added a master volume atfter C18, gave up on the cheap ceramic 8 inch. I ordered a Weber Alnico Blue Pup 10 inch with no dope. Used left over birch plywood, tolex and grill cloth from the AC15 and built a combo cabinet. I have about $140 in it now. If it craps out I will build a real hand wired AC4 for this cabinet and call it "Mini Me".
Now it needs some playing time to brake in the speaker.