6.6k or 7k primary OTs

[crgfrench]

Has anyone tried lower impedance primary OTs in the 18watters? Like 6.6k or 7k into 8 ohms? I might try one or two but curious if anyone has impressions of the sound?

[colossal]

I have run a quad of EL84s at 6k6 (Trainwreck Liverpool) and a pair of EL84s at 4k (similar to the intentional Matchless mismatch found in the Spitfire with the TDS output transformer wound to 4k). The difference between an 18W 4k voicing for a pair and the standard 8k is definitely audible. The power output is reduced and with it goes some volume. That's not a bad thing for some. The main difference in sound was thickening to the low end which made it more of a grunt. It was good but I greatly preferred the much wider, chimier, shimmering sound of 8k. You just get so much more "information" running 8k into a 16Ω speaker (my preference). I can get plenty of low end running at 8k. The 4k vs. 8k sound, to my ear, is similar to running an 8Ω speaker on an 18 watter. To me 8ohms sounds darker and loses the 3D depth and delicate top end and brilliance/presence. As for the Liverpool, running a quad at 6k6, that is not really an apples to 18 watts comparison. You could hear its Voxy AC30 DNA with the chewy, midrange-forward sound. Running a pair of EL84s on 6k6 or 7k, I'm not sure you would hear much difference, all other things like operating voltages being the same. That might depend on the type of OT though too. So if you use a full blown block of laminations the size of a JTM45 OT running a pair of EL84s on 6k6, yeah, you're probably going to pick up some headroom and a different tonal flavor.

I just finished a two channel 18 watt 1x12 combo with a Radiospares OT (parallel triode and cascade channels) and the only way I could be any happier would be to solidify the initial punch and attack on the low E string just a hair more; that's it. That is likely a function of the 20W Heritage Greenback I am running as well as the nature of the EL84 itself. But when the amp is opened up, that low end solidifies and then it sounds like a freakin' piano. The separation and harmonic richness is magical.

[geoff 1965]

interesting!
i'd like to see some detail & pics of the combo you've just built.
did you cascade between both triodes of the same tube and what primary does that radiospares OT have?

[colossal]

interesting!
i'd like to see some detail & pics of the combo you've just built.
did you cascade between both triodes of the same tube and what primary does that radiospares OT have?

I'll try to post some photos. This one is a standard 1x12 combo loaded with a 16Ω 20W Heritage Greenback (killer speaker that has been very sadly, recently discontinued by Ipswich ). PT and OT are Mercury Magnetics. The OT is the interleaved Radiospares 8k with ultralinear taps (unused). I used separate, dedicated tubes for each channel. The parallel triode has a single input, into Vol and Tone (500p/0.0047u) and the cascade channel also has a single input, but a Gain control inbetween stages, then the Vol and Tone control. Each channel goes to one side of the PI. I did not incorporate a (post)PIMV as I find, for my uses, I can just set the amp's Volumes and do everything from the guitar....mostly set it and forget it.

The cascade channel has 120k/2k7/680n on V1a into a 1MA Gain pot, then 100k/680R/3u3 on the second stage; right off of the Cascade mod schematic that's been around here for years. Both channels sound really good although I use the parallel triode channel more. It is just the sound I like. I didn't use the usual 820R/47uF voicing though as I wanted a little more brightness.

The other day, for fun, I hooked it up to a 16Ω 4x12. While it sounded like "itself", I didn't like it at all! The 4x12 is too thumpy and directional and muddles and confuses the sound. Sounded mono. Back to the 1x12 cab and all that 3D almost-stereo magick is back.

chassis.png

[colossal]

Has anyone tried lower impedance primary OTs in the 18watters? Like 6.6k or 7k into 8 ohms? I might try one or two but curious if anyone has impressions of the sound?

Also, just to add a little more context, I did not by means of my long winded reply, intend to imply that all other impedances were bad and only 8k was good. No, not at all. At the risk of being branded a heretic by the Spanish Inquisition, I will say I love the 18W platform but am not a strict Marshall purist. These circuits respond to small changes and the results can be amazing. So I would certainly encourage you to cast some load lines, get some OTs and play around. The plate and screen voltages are really important when considering the primary. Each has a sound or quality and you might find something that really does it for you. That is very personal.

95% of the time I just play with guitar and cable directly into an amp. I've done a lot of experimentation to build an amp that works with the guitar and pickups I like and use (Les Pauls with unpotted, low strength HBs) to make the sound I want to hear. That is always a work in progress. My perspective changed greatly when I moved away from building amps with preconceived expectations of tone or limitations driven by a time honored schematic and just built an amp based on a loadline and then tuned by ear.

So, you might like a pair of EL84s running at 6k and can balance whatever it may or may not do for you with speakers or other circuit tweaks. I know that sounds very artsy-fartsy but there are lots of variables to play with. That offhanded statement amounts to tens of hours of soldering and listening.

I built a load line tool in Excel so I can mess around with those input variables to make some predictions. I used a model by Keuhnel to extrapolate pentode grid curves for any screen voltage.

https://www.ampbooks.com/mobile/books/power-amps/

[geoff 1965]

you are lucky, the "monty python spanish inquisition mob" were just on their way round to see you!
seriously i like your "long winded" replies and it's good when people post their results from testing/tweaking so others can learn from them.
looking forward to seeing some pics of the amp.

[colossal]

you are lucky, the "monty python spanish inquisition mob" were just on their way round to see you!

Hahaha, right! Nobody expects the Spanish Inquisition!

seriously i like your "long winded" replies and it's good when people post their results from testing/tweaking so others can learn from them.
looking forward to seeing some pics of the amp.

Thanks man. I killed my phone recently so I can't take photos at the moment nor upload the ones I took

[crgfrench]

That's great feedback, thank you!

Anyone tried 10k or higher?

Also for the 8ks, I wonder if anyone has a preference for the Drakes or Dagnalls vs the RS? I know GDS offers the HTS repro version of RS but I might try a Drake repro or an actual Dagnall for my next build.

[colossal]

That's great feedback, thank you!

Anyone tried 10k or higher?

A Liverpool running 4xEL84 at 6k6 is the equivalent of 13.2k for two EL84s. I've also run an 18W with a 16Ω speaker on the 8Ω tap, effectively doubling the primary to 16k. Doing so rotates the load line down below the knee of the grid curves. This favors earlier breakup and a little lower output. It creates some compression as the more flat the load line becomes, the more uneven the spacing between the grid curves become. This approach has the downside of pushing screen conduction into dangerous territory on big signals, so screen resistors should be upped and more voltage dropped to the screen supply (this also lowers the knee back down to compensate for the shift). This is why the Trainwrecks use that big 1k 25W "choke" resistor. There is massive rail sag when there is a big transient and the whole thing compresses. This is what gives the evenness in sound level when going from clean to mean; the whole Trainwreck thing. As to how it all sounds?? All tube amps compress when pushed and that's why we like them. I prefer a more open and dynamic sound and don't care much for massive amounts of preamp distortion or overly compressed anything.

Also for the 8ks, I wonder if anyone has a preference for the Drakes or Dagnalls vs the RS? I know GDS offers the HTS repro version of RS but I might try a Drake repro or an actual Dagnall for my next build.

I have no experience with the Drakes or Dagnalls (aside from big Marshalls). I am very much a fan of the RS transformers though. They are interleaved and use M6 steel.