1965 JMI Vox AC30 TB

A Sept 1965 Super Twin Trapezoid head amp, rescued from eBay. [Updated for clarity]

I had wanted an AC30TB for many years, but they were aways hard to find and kept getting more pricey by the year. I finally found a battered broken hulk of one on eBay one day and outbid a guy in Florida for it. I know where he was because he reached out to me after the auction to find out what I was going to do with it. It was a painful $900 for a broken amp in rough shape that needed a lot of work, but I was finally in the game. It was completely random, I just bumped into it. The key for me was that the output transformer and choke were original; in an amp circuit with no negative feedback you are going to hear the output stage and speakers pretty directly.

I had heard the trapezoid heads from the mid 1960's were the "best" based on the experiences of some studio engineers I knew. They told me stories about the one belonging to Chris Butler (of the Waitresses fame) although they thought it was a trap shaped AC50 - and those never existed. So I knew I was looking for an AC30. I also knew AC30s came with an array of different transformers over the years; RS, Haddon, Parmeko, Albion, Woden... it was dizzying trying to understand which were better/worse and when they were used. Everyone had an opinion on which was "the one" to have, of course. (usually the one they had!)

The original power transformer was long gone and had been replaced with a pretty ugly kludge that put out the wrong voltages and didn't mount properly. I got on the phone and asked Heyboer if they could wind up one to my specs and they said yes. The part number is HTS-7047 if you ever need one. First off, it mounts properly. Second, it is designed to deliver 250mA at a B+ of 330V, and then power for the tube rectifier & 6.3V filaments. This puts 330V on the plates (less 10V on the cathodes for 320V operation - the Mullard spec) These amps idle at 200mA which has always made me scratch my head, because the factory schematic seems to indicate that the power transformer is delivering 160mA - well below what is required. And that would make some sense since so many of these amps seem to run the power transformer very hot, or just blow them. My goal was to have one that would not get hot or fail. Call me crazy. (get in line)

I got the power section repaired and the amp "worked" but sounded pretty noisy & rough and the trem did not work. Luckily a fellow I know from my amp circles actually lived in the same town as me and as luck would have it, was actually around. He was a radio officer in the merchant marine so he was away a lot. In any event, he is lot more technical than I am so I gave it to him to get it into basic shape. I knew I was probably over my head.

I got it back a few weeks later, working a lot better and much more quietly. And now I could focus on playing and minor maintenance. The heads are known as "Super Twins" and this one had top boost incorporated on the control panel as opposed to being an add-on in the rear. A very cool amp! They originally came with a chrome trolley and a sealed 2x12 cabinet. Probably a real pain to move around, but very striking looking. The set that came with this head was sadly long gone.

A pretty handsome combo if I do say so myself. I only found the head, and it was pretty rough.

Wrong label, right amp. 3 of the vents are missing. If I can find enough fellow crazies I will see about getting more made. I need an excuse to buy a used electric blast furnace.

I had this 2x12 cab made to the size of a blues breaker. I standardized on 16 ohm cabs on the theory that they run less current into the speakers, use the whole secondary winding and finally because it would work with both the AC30 as well as the JTM45 I was building. I gigged with this pair for close to 15 years. The larger cab made the amp's voicing a bit bass heavy though.

rear view.

One odd bit about the head cab is this opening that was clearly for the add-on top boost. It had a piece of leather covering it. This amp chassis has TB on the panel so was this a cabinet swap? The chassis is for a trap head, like the AC50 they have different mounting points for the different head types, and here it is head vs combo.

Now it lives in a Northcoast cabinet with a pair of 60's Celestion Silvers that were re-coned by Vin-Tone, and the cabinet size really tightened the sound up. Harder to carry (75 ish pounds) but easier to gig with since I can make one trip to the car.

The Vox family. The rearmost head is one of Stephen Walsh's 715 tribute amps.

Dating

I wanted to figure out roughly when it was made to know if the cabinet was original or not. And I still don't really know, but it probably is. Here are the dating details I was able to find out:

(this is all per the wonderful Jim Elyea book)

* Slant heads were made only in 1965-66. Apparently the cabinet shop hated them.

* Output transformer is Woden 76853 (11/64 to end of JMI.) Date code "HW" = Aug 65

* Choke appears to be a Parmeko 241/DT (possibly replaced? bent tabs)

spec appears to be correct - 20H, 500 Ohms

* Chassis SN is in first shared sequence (up to July 64?) 12604. (12911 is last of the original group)

* Backplate SN is May/June 65.

* Pot code: GM = July 65

* Mullard Cap code: D4M = 4th qtr '64 (M or N indicates the factory.)

* Pilot Light: correct type for period

* Fuse Holder: correct type for period

* Voltage Selector: correct type for period (post Nov 64)

* On/Off switch: correct type (mk II)

* Resistors: White Erie mostly.

* Caps: Mullard "Mustard", Wima primarily.

Mullard Date codes etc: https://mullard.org/blogs/news/d

"The C296 series of capacitors also carry a three digit batch code of the form A8B

Where digit 1 is the quarter code: - A = January - March; B = April - June; C = July - September; D = October - December.

Where digit 2 is the year code: - 1 = 1961; 4 = 1964 etc....

Where digit 3 is the manufacturing plant: - B = Blackburn; D = Hamburg: S= Barcelona;"

These show as D4M & D4N, so Q4 1964 but made where? lol

Einstein teaches us that the amp can't be older than the most recent component, which here is the OT. That is dated August, and so I usually tell people that the amp is a September '65. The transformers were the most expensive components and it is likely that there were not lots in inventory sitting around. It is interesting that the SN on the plate is for May/June production. Was it built up and waiting for the final component(s) or waiting for an order?

The slider boards (what the amp is screwed on to) are different for the trapezoidal heads than for the square heads & the combos, so that also seems to point to this all being from the same package. The trap head board is not as wide.

Original Woden 76853 Output transformer

Chassis SN (not amp SN!)

Stock cathode resistor of 47 ohms ran the amps very very hot. Very, very, very hot indeed.

The chassis and the Heyboer HTS-7047 power transformer. The factory spec shows a 280V secondary which in theory could hit 390V (!). The HTS-7047 has a 240V secondary and hits 342V at the OT center tap. This yields 334V at the plates (my design target was 330V), 325.5V at the screens and with a 45.6 Ohm Rk (marked 47) it biased at 9.9V which is 217mA, or 54 per tube. That is 17.6W at idle for a tube that is rated to dissipate 12 watts. So I also fixed the cathode resistor. That brought our idle down to 40mA per tube or 13.2W which when we subtract screen current works out to about 11.8W, and the EL84 is rated at 12W. <phew!> It is also right around that 160mA spec on the Schematic.

Here is that section of the factory schematic, indicating 10V of bias at idle (voltage on the cathode) 10V across 50 ohms is V=I*R or 10 = I * 50 = 200mA. You can also see the power transformer secondary where it appears to indicate 160mA and 280V-0-280V. Which I will go on record calling "insanity". 280 * 1.414 = 395V less any losses in the rectifier tube. (insanity)

Also, not to belabor a favorite point of debate among amp geeks, but the fact that the bias shifts (from 10V to 12.5 at full output - +25% - indicated here on the factory schematic!) Tells you that the amp cannot be class A because the current would not change if it was. The bias is shifting up because of self rectification from running in class AB. Fixing the bias brings the amp down to around 160mA of draw (the signal triode sections are pulling around 1-1.5mA each, not really impactful) which is a much better place.

Metal cabinet vents (not plastic). There are actually 2 different parts, one with the logo (shown) and one without.

A broad gut-shot of the upper chassis. Most of that is the Vib/Trem circuit. The amp itself is just the far right of each tag strip. The lower chassis houses the power supply and output circuit.

Stuff we fixed or updated

New rectifier socket for the GZ34, the new wiring is from the new power transformer. And we added rectifier diodes to the socket to protect the tube.

We re-built the output section, using larger grid resistors and screen grid resistors as well as the 68 Ohm cathode bias resistor that the earlier amps had used. The 47 Ohm had the tubes idling at 17.6W / 54mA per tube, and the 68 ohm lowered that to 13.2W / 40mA per tube. Subtract the screen current and we're now at 11.8 Watts. The EL84 is rated at 12W.

We should be a lot more reliable, and not burn through tubes. You do not need to run amps on the edge of melting to sound good.

Looking for out of spec resistors with a DMM. I also replaced electrolytic caps like the cathode bypass caps.

Using a dental pick to open up a hole where we're getting ready to install a new in-spec part. Working on these is pretty tight.

New components in the phase inverter (resistors and coupling caps)

New parts in the pre-amp. There were some truly crazy resistor values in there that were killing the clean sound. Now the amp is strong and clear and still has great break up.

Just for fun, here are the resistors in the TB channel that got the boot for being crazy:

A 220k at 295k (first stage plate)

A 100k at 156k (2nd stage plate) A 56k at 85.3k (2nd stage cathode follower)

Replaced the cathode bypass cap on the first stage Replaced the treble cap (47pF 1000v silver mica) in the tone control circuit

The screen resistors were all over the map, as were grid stoppers on the output tubes so they got fixed as well. It made a huge difference.

The amp was pretty roughed up as I had mentioned; I needed to replace a pot or two, and move some leads away from the front panel that would short the sound at weird times. I also needed to replace the input jacks which did not make reliable contact anymore. Not a huge job but details...

Label the 3 shielded input cables, and pull out the 6 jacks as a unit.

Use the chassis as a jig to build the new ones. I bussed them together the same way the originals were. I used some 16ga filament wire for this.

All ready to solder and re-install

June 2023 Update:

The amp runs well but still has a bit of "whack-a-mole" left in the old coupling caps. So every now and then the sound gets weird and I have to go find and replace a coupling cap. I am tempted to replace all the couplers in the Normal & TB channels just to stop having to worry about this when recording or gigging.

Special thanks to: Ray Ivers, Lyle Caldwell & Alan Vale for their expert counsel, mad skillz and moral support. :)