Heyboer HTS-7881

[Stevem]

Looks good according to the RCA tube manual.

[Fischerman]

Oldish thread but I was just piddling with this thing today. I bought the transformer set from APD and I wondered about the impedance mismatch. I used a Fender type choke. My understanding is that it is the same OT as used in their 30W amps. Using it at 'regular' output impedance setting it is a strong, powerful amp for 2xEL84s. Too much bass imo. I changed the 22k screen node dropper to 10k which puts the screens at 290vdc in my amp. Plates are 350vdc. Cathode resistor is 120R and is at 10v. I'm using the Russian 14W tubes. It sounds great but I did several mods. Namely, 220k V1 plate reduced to 110k and a switch (Bass control pull pot) to switch in/out the cathode bypass cap on V2, dual-pot post-PI MV and added a Cut control.

[Colossal]

Oldish thread but I was just piddling with this thing today. I bought the transformer set from APD and I wondered about the impedance mismatch. I used a Fender type choke. My understanding is that it is the same OT as used in their 30W amps. Using it at 'regular' output impedance setting it is a strong, powerful amp for 2xEL84s. Too much bass imo. I changed the 22k screen node dropper to 10k which puts the screens at 290vdc in my amp. Plates are 350vdc. Cathode resistor is 120R and is at 10v. I'm using the Russian 14W tubes. It sounds great but I did several mods. Namely, 220k V1 plate reduced to 110k and a switch (Bass control pull pot) to switch in/out the cathode bypass cap on V2, dual-pot post-PI MV and added a Cut control.

When you say, "Using at 'regular' output impedance", are you meaning doubling the impedance to 7.6k? Do you normally run at 3.8k? I think I recall you saying in a different thread that running this amp at 3.8k for two EL84s, the low end rivaled that of your 30W Rocket.

I have always wondered about the Matchless Spitfire and Lightning running a pair of EL84s at 3k8. Here is where I am seeing your operating conditions.

Fischerman.png

[Fischerman]

Oldish thread but I was just piddling with this thing today. I bought the transformer set from APD and I wondered about the impedance mismatch. I used a Fender type choke. My understanding is that it is the same OT as used in their 30W amps. Using it at 'regular' output impedance setting it is a strong, powerful amp for 2xEL84s. Too much bass imo. I changed the 22k screen node dropper to 10k which puts the screens at 290vdc in my amp. Plates are 350vdc. Cathode resistor is 120R and is at 10v. I'm using the Russian 14W tubes. It sounds great but I did several mods. Namely, 220k V1 plate reduced to 110k and a switch (Bass control pull pot) to switch in/out the cathode bypass cap on V2, dual-pot post-PI MV and added a Cut control.

When you say, "Using at 'regular' output impedance", are you meaning doubling the impedance to 7.6k? Do you normally run at 3.8k? I think I recall you saying in a different thread that running this amp at 3.8k for two EL84s, the low end rivaled that of your 30W Rocket.

I have always wondered about the Matchless Spitfire and Lightning running a pair of EL84s at 3k8. Here is where I am seeing your operating conditions.


Fischerman.png

No I meant running it at the labeled 4/8/16 ohm. And yes it did rival the Rocket in low end when built to Lightning specs. And switching the impedance helped but didn't cure it.

I think I'm trying to get AC15 tone from a Lightning and in hindsight a Lightning is too 'hot rodded' for my taste. If I want high distortion I'll use other amps/pedals. So I've dialed it back (no cathode cap on V2 is 'default' mode, pull boost engages cap) and leaned it out a little.

It sounds really good, even when cranked into distortion. It's not fuzzy or super fizzy, just nice and grindy. I did do the zener mods on the cathode and grids (two different mods) but my bias is 10v and the only zeners I had were 15v so I'm not sure they're doing anything.

[Fischerman]

I have always wondered about the Matchless Spitfire and Lightning running a pair of EL84s at 3k8. Here is where I am seeing your operating conditions.

What is the dotted redline?

[Colossal]

I have always wondered about the Matchless Spitfire and Lightning running a pair of EL84s at 3k8. Here is where I am seeing your operating conditions.

What is the dotted redline?

The red line is the Maximum Dissipation Limit.

Thanks for clarifying my impedance question. When I spoke to Heyboer about this OT years ago (it was originally a teardown of a West Labs OT which was a copy of an original Matchless OT) they had nothing to offer as far as why Matchless would be operating a pair of EL84s like this. Looking at the loadline it looks like "current mode" where the voltage change is quite small but the current change is large, as compared to a typical Class AB amp. Your tubes idle at 42.3mA apiece which is reasonable and typical of a Lite IIb and less than an AC15 (48mA). I have read internet lore which said "Matchlesses eat tubes" and others which disagree, and you might think that operating a pair of EL84s deep into the red zone, like the loadline shows, would make for a short tube life. Of course, tubes were more robust in days past. Martin stated that lowering the screen voltage would make this worse, so I'm not sure what to make of the Lightning schematics; one of which shows the screen voltage taken directly after the choke, and another showing a 22k dropping resistance and cap, then the screen node. Trinity Amps was following the latter but their OT impedance for 2xEL84 was 5k, not 3k8. You mentioned settling on a 10k/cap screen node after the choke and this gives a screen voltage of 290V with a plate voltage of 350V.

[Fischerman]

Your tubes idle at 42.3mA apiece which is reasonable and typical of a Lite IIb and less than an AC15 (48mA).

I just remembered that I built this amp with a particular quad of 'hot' 6p14p tubes that have extra large plates, even when compared to other 6p14p tubes I've had (about two dozen).

So if I put a typ pair of EL84s it wouldn't run as hot as it does.

[flemingmras]

Oldish thread but I was just piddling with this thing today. I bought the transformer set from APD and I wondered about the impedance mismatch. I used a Fender type choke. My understanding is that it is the same OT as used in their 30W amps. Using it at 'regular' output impedance setting it is a strong, powerful amp for 2xEL84s. Too much bass imo. I changed the 22k screen node dropper to 10k which puts the screens at 290vdc in my amp. Plates are 350vdc. Cathode resistor is 120R and is at 10v. I'm using the Russian 14W tubes. It sounds great but I did several mods. Namely, 220k V1 plate reduced to 110k and a switch (Bass control pull pot) to switch in/out the cathode bypass cap on V2, dual-pot post-PI MV and added a Cut control.

When you say, "Using at 'regular' output impedance", are you meaning doubling the impedance to 7.6k? Do you normally run at 3.8k? I think I recall you saying in a different thread that running this amp at 3.8k for two EL84s, the low end rivaled that of your 30W Rocket.

I have always wondered about the Matchless Spitfire and Lightning running a pair of EL84s at 3k8. Here is where I am seeing your operating conditions.


Fischerman.png

That goes well and above 2x Wa, which will lead to overdissipation. Also not very efficient power transfer. I suppose if the power supply sags enough you wouldn't even need to mismatch.