Vox AC30/6 TP Bias Question

[opacheco]

Hi

I am in the repair process for a VOX AC30/6TP guitar amp and I would like to be clear in some questions:

1-Is this amp a CLASS A tube amp or Push-Pull Amp??,
If this amps is a Push-Pull amp;
2-What is the best Bias for performance and Rock Music for each tube??

Thanks a lot anticipated,
Opacheco.

 

[CJ]

it is probably a Class A Push Pull amp,

is there a bias pot or is it cathode bias? (like a 250 ohm power resistor connected to both cathodes)

 

[firehazard]

Hi,

an AC30 is cathode biased.

Depending on the year it was made the cathode resistor can be from 82ohm to 50ohm.

This resistor is shared by the four EL84 output tubes.

50ohms runs the tubes at a higher power output. Some people prefer the sound but tube life may be reduced. The modern chinese made AC30 could switch between these values.

The circuit has no negative feedback loop and this might be more important to its sound than whether it is Class A or AB1.

Take care

 

[opacheco]

Hi,

an AC30 is cathode biased.

Depending on the year it was made the cathode resistor can be from 82ohm to 50ohm.

This resistor is shared by the four EL84 output tubes.

50ohms runs the tubes at a higher power output. Some people prefer the sound but tube life may be reduced. The modern chinese made AC30 could switch between these values.

The circuit has no negative feedback loop and this might be more important to its sound than whether it is Class A or AB1.

Take care

firehazard,

Thanks for your information, that's very clear!!
By the way, do you knows what is the idle plate/cathode current?....any idea??

Opacheco

 

[firehazard]

Hi,

both of the books I have as references for my own AC30 assess the operating point of the output stage by measuring voltage across the cathode resistor. This seems to be the traditional approach for these amps.

The values given by "A Service Engineer's Guide to the Vox AC30 Valve Amplifier" are probably the ones I would trust most.

10 volts quiescent and 12.5 volts at 30 watts output. This information is for a 50R cathode resistor.

I mentioned earlier that these amps run the outputs hard. Probably above the values given in the original EL84 datasheet. Russian military valves are probably the better choice these days. Certainly cheaper.

Take care.

 

[opacheco]

Hi,

both of the books I have as references for my own AC30 assess the operating point of the output stage by measuring voltage across the cathode resistor. This seems to be the traditional approach for these amps.

The values given by "A Service Engineer's Guide to the Vox AC30 Valve Amplifier" are probably the ones I would trust most.

10 volts quiescent and 12.5 volts at 30 watts output. This information is for a 50R cathode resistor.

I mentioned earlier that these amps run the outputs hard. Probably above the values given in the original EL84 datasheet. Russian military valves are probably the better choice these days. Certainly cheaper.

Take care.

firehazard,

Ok thanks for your notes!!....
Opacheco.

 

[CJ]

you really hear the tubes in that amp,

problem is the orange script Holland Bugle Boy 6BQ5's are about 200 bucks a pair if you can find them,

but they sound 10 times better than anything else you stick in there,

God Bless Charles Kittleson  :'( 

http://www.vacuumtubevalley.com/Magazines/VTV08a.pdf

 

[firehazard]

Hi,

that was an interesting read. Thanks CJ.

The cost of keeping a stage amp in full vintage glory is beyond me. Four tubes would cost more than I paid for the whole amp. So it's 6p14 ev's for me at 60euro or so for all four.

take care

 

[CJ]

that article cam out before the JJ EL 84, so i wonder how it would sound,

anybody try the JJ EL 84, give us a review, thanks,

 

[pH]

Good read, but yeah, probably a little outdated. Tubes from Russia have gotten much better since then. I've used JJs a bit in the past, sounded good but I never did a real shoot-out against other tubes. My Amperex pulls do seem to have a little special sauce though.

Regarding biasing, you need to calculate bias in relation to plate voltage, simply measuring cathode voltage isn't enough to figure out plate dissipation. Try using the Weber bias calculator: http://www.webervst.com/tubes1/calcbias.htm

Spec sheets for EL84 specify 12 watts max plate dissipation, vox likes to push it up to 16. I usually bias a bit cooler than that, doesn't really seem to hurt the sound and won't cook the power tubes so quickly.

 

[CJ]

you can drop the screen volts down 20 percent and add some life to your pwr tubes,

 

[Bo Hansen]

I'm repairing and renovating about 5-10 pieces old AC-30 each year. (and some 50 others with EL84)

Have tried many of newly manufactured Russian and Chinese EL84/6BQ5.
Many of these will be fairly soon microphonic and comes into resonance with speaker sound.
This also includes some other amplifier contains EL84, such as the Fender Blues Junior, Peavey Classic, etc.

In my opinion, the best new manufactured EL84 that works well in similar amplifier is JJ from Slovakia.

Of course, the old brands like Philips, Mullard, Telefunken, Siemens is usually best, but it's hard to find a "NOS" pair or quartet that is matched.

--Bo

 

[firehazard]

Hi,

I think the change in 1963 to a 50R cathode resistor was more for marketing reasons than good electronic practice. The AC30/4 used 80R and is a fabulous sounding amp apparently.

The engineering in these amps is marginal at best. This seems to be normal for great sounding amps made in England during the 60's. There are a few Marshalls that run output transformer impedances so low that redplating is only avoided because the B+ collapses under load.

I suspect a vintage AC30 will not have its nominal 320volts when it's giving up the "30 watts" either. This might explain the traditional approach to checking the bias?

I doubt Dick Denney was thinking that we would still be second guessing him fifty years later.

Take care

 

[CJ]

thanks Bo, good to see ya! 

yes, the engineering, well, if you look at the top boost mod, kind of a kludge,

i like the pentode input tube channel the best, but the tube needs a lot of TLC,

shock mounting the socket and reducing screen volts reduces the gremlins,

 

[firehazard]

Hi,

all I meant by my comment was that these amps might not follow orthodox thinking or stand up to "textbook" analysis.

The design seems a great example of "Who cares what it says in a book? Listen to this!"

Too many great sounds have been pulled out of these over the last fifty years to start worrying about how it works in theory......

take care

 

[CJ]

they are great amps, but like a race car engine, ready to blow up at any moment,

speaking of fire hazard, a few of those  AC 30's caught on fire as chassis air flow is limited, 

if you want a killer sound, try a Fischer output transformer 7.5 K:8/16 made for 7591 tubes, done on a rare 91 EI lam, has 1005606 stamped on the end bell, available out of old Fisher stereo amps,

 

[firehazard]

Hi,

thanks for the transformer information and your thoughts on screen voltages. I have a small single ended amp I might try an EF86 in. Suitably shock mounted.

You are right with the fire hazard reference. I have a few clones in mind.

take care

 

[opacheco]

Hi,

both of the books I have as references for my own AC30 assess the operating point of the output stage by measuring voltage across the cathode resistor. This seems to be the traditional approach for these amps.

The values given by "A Service Engineer's Guide to the Vox AC30 Valve Amplifier" are probably the ones I would trust most.

10 volts quiescent and 12.5 volts at 30 watts output. This information is for a 50R cathode resistor.

I mentioned earlier that these amps run the outputs hard. Probably above the values given in the original EL84 datasheet. Russian military valves are probably the better choice these days. Certainly cheaper.

Take care.

firehazard,

I replace all the pots in this amp and changed the rectifier tube for a Mullard GZ34 and the powers tubes for a Tung Sold quartet matched with 47 miliamperes of bias. The issue that I have now, is the voltage in R70/R71 (both 100ohms and 5watts) goes up from 10.9 DCVolts to 11.6 DCVolts and when this happens, the plates become almost red after 4 or 5 minutes (I turn off the bias switch before the tubes goes really red, before of the tubes death of course!!). According to my electronic analysis, the bias current in each power tube (with R70/R71 both 100ohms), is 50 milliamperes; and 47 milliamperes will be fine, I thought!!....but the reality is not!!!.

I am thinking to increase the value of R70/R71 from 100 Ohms to 120 or 150 Ohms in order to get 60 -75 Ohms (parallel circuit) and reduce the polarization current; I would like to know your suggestions about!!......any comment or advise will be great for me.

Thanks anticipated,
Opacheco

 

[CJ]

what was the model of the old rectifier tube?

if i remember right, they used a 6X4 or something weird like that,

GZ34 is known for it's low forward drop, so your B+ might have gone up

this could  be why the plates are red,

 

[MagnetoSound]

GZ34 is correct.

Brand seems to mean very little these days, red-plating is not at all uncommon even with tubes that purport to be NOS. These Tung-Sol could have been made any time, anywhere and simply printed up with the name later; I would not trust them. If the plates glow red at the correct bias current just replace them with a set that works.