EL844 Amp Based on 5f6a?

[The New Steve H]

Now that I'm a certified amp expert with three amps functioning, I am thinking it would be great to try to do something original.

The guys at my church complain that their Vox AC30 amps are too loud. They can't be cranked up to the point where they sound good. I just completed a 5f6a Bassman clone, and I was very surprised at how good it sounded at low volume, and how useful the volume control was. That made me wonder: would it be a good idea to make something similar to a Bassman, with a twin EL844 output stage? I'm thinking maybe I would end up with something a little quieter than the Bassman, with the same virtues.

I would want to get rid of the standby switch and work hard on grounding and shielding, in order to minimize noise. And I'd want to keep the 12ay7 preamp tube. I don't know anything about VVRs, but if it's possible to wire one in, that would be a nice touch.

I'm too cheap to make combos, so this would be a head. I would definitely want to have a tube rectifier so I could experiment with sag.

Does this sound doable? I assume the preamp and tone stack would be just like the Bassman, but I would have to figure out how to combine them with a different set of output tubes. I'm not sure how I'd go about choosing transformers.

[M Fowler]

It's just an the 18w TW Rocket with the 5F6a tone stack.

Mark

[sliberty]

An amp built on 2 EL84's could make use of an AC15 or a Marshall 18 Watt output stage. Take a look at the schematics for those amps, and buy appropriate transformers based on their designs.

By the way, did you consider installing a VVR in the AC30's? Of course if these are vintage pieces, I wouldn't do it, but if they are modern AC30's, it is an option.

[10thTx]

I played one of these in a church setting a few times and it sat just fine in the mix and we mic'd the speaker. We had 3 singers, a LOUD drummer, another guitar and keyboard.

Still cut thru the mix just fine. Mine was similar but had some different component values. Only 7-9 watts

Editable layout and schematic are here:
http://www.el34world.com/Forum/index.php?topic=562.0

With respect, 10thtx

[billyz]

I just modded a Jet City 20 into something pretty close to what you are talking about. It turned out great.The preamp gain and master
allow for good fat distortion at even lower levels. I would think it would be a good candidate for what your church guys need.
Z

[The New Steve H]

Fantastic info. This gives me somewhere to start. I'm glad I built the 5f6a. I did not understand what a great amp it would be.

[billyz]

Here is a Schematic of the preamp mod I did to the Jet City 20.

The power amp is an el 84 pair adjustable fixed biased . Good trannys to start with too.

[The New Steve H]

the Mini Bassman is neat, but I don't think I want to go below 15 watts of output. For home use, though, it would be great.

[jestaudio]

[quote="billyz"]I just modded a Jet City 20 into something pretty close to what you are talking about. It turned out great.The preamp gain and master
allow for good fat distortion at even lower levels. I would think it would be a good candidate for what your church guys need.
Z[/quoteI tried a jet city a few weeks ago and loved the sound but had trouble with clean headroom, does your mod affect the clean headrom

[M Fowler]

The parallel triodes V1 will definitely increase gain but might need to keep C15 as is for clean?. That 25uf/25v bypass cap would bring on even more gain.

What is nice about this amp is that is is very easy to try different caps to suit to taste.

Mark

[billyz]

The parallel triodes V1 will definitely increase gain but might need to keep C15 as is for clean?. That 25uf/25v bypass cap would bring on even more gain.

What is nice about this amp is that is is very easy to try different caps to suit to taste.

Mark

Look carefully at the Parallel triode, the plates are 50k total and the cathode is divided in half as well. Gain does not go up. If I had used only one 100k plate it would have increased. Much increased clean headroom this way.

[M Fowler]

Billy,

Yes I did wonder about that after I wrote my message. I was looking at the change note there too, 100k in red.

Mark

[billyz]

Billy,

Yes I did wonder about that after I wrote my message. I was looking at the change note there too, 100k in red.

Mark

Mark, I actually did play around with C15 ( V2/a ) it did not increase the gain as much as fatten out the bottom end. I could have left it either way, but preferred the warmer fuller tone of the 25uf. Same for bypassing R53, it increased the volume a bit but also brought up more detail , harmonics, less 2d sounding. different than just turning up the volume to compensate.

[The New Steve H]

I took a look at Trainwreck Rocket documents. Doesn't look too much like a Bassman. I'd be afraid I'd get it finished and have something that sounds different. Is that crazy?

It looks like the Tweed Deluxe has the same preamp tubes as the 5f6a, but the output is 15 watts. What if I start out with the same preamp as the Bassman, no changes, get a Tweed Deluxe power transformer and output transformer, and put together an EL84 or EL844 output stage? Is it more complicated than that? The voltages on the Tweed Deluxe PT are the same as the Bassman transformer I used.

http://www.magneticcomponents.net/40-18021_Stock.pdf

The only thing that worries me is whether transformers from a 15-watt amp are big enough for an amp with two tubes that dissipate 9w each.

[diagrammatiks]

el84s and 6v6s need transformers with different primary impedances.