Design considerations for Vox choke choice

[Bob M.]

I'm wondering why Vox amps - now I'm thinking older Vox amps here, like the AC30, AC15 and AC10 - use such massive chokes, especially as compared to the older Fender amps of the same era. Now, most older Fender amps used one of two different chokes (from Twins to Deluxes) and both were quite small compared to the ones seen in Vox amps. What were the design reasons for this? I know most companies will use the smallest, cheapest part that will do the job, generally speaking.

Bob M.

[g1]

Most of the current taken from the supply is used by the power tubes. If you want to have that part of the current filtered by the choke, you need a big choke. The vox's you mention do it that way. Fender takes the feed to the power tubes before the choke, so they only need a smaller choke to run the rest of the circuits.

[soundmasterg]

Most of the current taken from the supply is used by the power tubes. If you want to have that part of the current filtered by the choke, you need a big choke. The vox's you mention do it that way. Fender takes the feed to the power tubes before the choke, so they only need a smaller choke to run the rest of the circuits.

What are you talking about? Looking at an AC30 schematic, and an AC100 schematic, Vox is supplying the power tubes before the choke just like Fender. The choke tap supplies the screens. Maybe you're thinking of a Sunn 200S/2000S or similar?

Greg

[g1]

I was looking at this AC15. He mentioned AC10 and AC30 as well, I assumed there were maybe 2 versions of AC30, one with the OT tap after choke but I guess that is incorrect.
So let's remove AC30 from the discussion.

[Chuck H]

Right. What g1 said. So... If that big choke is what Vox used it could well be because they had them.?. I wonder if it's the same choke in the choke fed plates AC15 as the choke fed screens AC30.?. That would explain a lot. Quantity discount? Simplify inventory to one part? etc.

[g1]

I guess it's also possible Bob M.'s question was more about what are the advantages (sonic) of having the larger choke feeding everything. If there were not an advantage, you would think they would use the cheaper option/smaller choke.

[Chuck H]

For a modern guitar amp I don't see an advantage. Most of any hum is cancelled in the push pull with matched tubes (and they all are now?) and part of the charm of guitar amps is the bouncing voltages in the power amp affecting dynamics. Some of that gets lost with a choke feed. JMHO

[pdf64]

Pentodes tend to be more sensitive to HT ripple than beam tetrodes; their ratio of g2 current to plate current is higher and I suspect their g2 mutual conductance tends to be higher.
Yes the ripple fundamental tends to get cancelled out in p-p but too much and a ring modulator effect will be produced, due to sum and difference frequencies of the ripple and signal being generated.
A touch of that may possibly act to thicken the tone but more than that and it will likely just sound horrid.
Even with a high value choke, Vox (and some Marshall) HTs bounce around plenty! http://www.hammondmfg.com/guitarLineCHK.htm

A 'side' benefit of high value chokes for the screen grid node is that their higher resistance can act to limit screen grid current under high signal conditions, especially overdriven, and especially for pentodes.

[Chuck H]

A 'side' benefit of high value chokes for the screen grid node is that their higher resistance can act to limit screen grid current under high signal conditions, especially overdriven, and especially for pentodes.

By resistance, do you mean impedance? Most chokes, even big ones, only have DCR in the hundreds. A choke with a high inductance though could rival the OT for impedance and that surely would mitigate G2 current better than a 470R or 1k resistor. It could also relieve the need for individual screen resistors which would help the screens voltage track faster and more accurately at higher DCR. Likely making the amp sound bigger and more dynamic. I've heard it mentioned that the AC30's are really very loud for a quad of el84's. I always chalked it up to the speakers characteristics making them apparently louder. Maybe it's actually that big choke and those little 100R screen resistors compared to everyone else's 1k resistor in the rail and individual 1k screen resistors .?.

[pdf64]

I was thinking of the choke's resistance (~500 ohms on a 60s AC30) rather than impedance, as under heavy load, I think it's the resistance that will make the operating conditions a bit easier, as the average Vdc at the screen grid HT node will come down. Plus heavy current draw will probably reduce the choke's inductance, perhaps significantly if it saturates.
The 1k dropper on the TW Express has a massive effect, to the point of causing the LTP to clip the signal (ie just running out of signal swing), rather than the power tube control grids. There's a thread over on TAG with an in-depth investigation of this.
Screen grids can draw a lot of current when overdriven, especially with pentodes

But I take your point, the little 100 ohm screen grid stoppers may help to keep plenty of punch, and under dynamic conditions, a lot of inductive reactance may well act as suggested.

[Chuck H]

The 2xel84 design I'm doing now uses a whopping 2.2k between the plates and screens. It runs about 365Vp and 330Vg2. I'm getting fifteen clean watts and with an efficient speaker that seems fine. Something I've seen quite a lot lately is g2 over dissipation during heavy g1 drive in vintage designs. It's looks pretty brutal when you see it. I've been battling it with the brute force method of increasing screen circuit resistance and reducing grid drive a little. A tiny bit of g2 overcurrent on the bench with a continuous input signal seems fine though because in real life playing conditions that's not going to happen. Probably why these amp designs have survived (somewhat) for so long. But old amps overdriven do occasionally blow up tubes. At least partly because of this. On the design I'm working on now I reduced voltage to the PI and am running the tail at 47k to eat up a little more swing. The power tubes are still well into clipping when the PI starts to clip, but the poor little el84 screens aren't over dissipating. It's starting to sound pretty damn good. I may experiment lower Vp and a choke for the next amp to see what happens at the screens now.

[pdf64]

The mystifying thing to me is how those old AC30s managed to sound so good, when on the face of it they should be a nightmare of blocking distortion.

[Chuck H]

I know! I often look at vintage designs and wonder. Even the TW express looks hinky in it's own way!?! Sometimes after some consideration and bench testing I get to see what's happening and then understand how it actually affects the tone in a positive way. But, of course, I'll never get to see all the amps I wonder about. The best I can speculate about the AC30 is that some grid conduction is responsible for the "swirl" and the output transformer and speakers are eliminating some buzz and fizz in the tone. Of course they DO buzz and fizz a little. All el84 amps seem to. But some do it horribly audible and others don't. Even clones of el84 amps that aren't known to sound fizzy sometimes buzz like bees and need to be corrected with something the Paul Ruby mod.

[daz]

Likely making the amp sound bigger and more dynamic.

I can tell you this much. My 6V6 amp had a 1k resistor and i have been playing and tweaking it for a few months and recently ripped the resistor out and replaced it with a 20H choke from another amp and the feel and dynamics and got much better. It's what i was missing from the other amps i built that had the same choke. The tone is huge too with a low mid wallop like never before. I'll let you guys debate why because of course i have no idea, but it was a revaluation. I used that choke in my EL34 amps before but i never used a resistor in those so i wasn't aware of how much difference the choke makes. Mine is positioned normally between plates and screens.

[SoulFetish]

Chuck and PDF, to your points, I've noticed for some reason parallel EL84s seem to suffer less from blocking and buzzing then single ones using the same driving circuit. I cant figure why yet, and if anything I would think the opossite would be true, due to the load impedance dropping twice as much during g1 current conditions. Its a little superficial of an observation because I haven't put it under test but I'm wondering if you guys have experienced this at all?