the fun begins !

[lovedat800]

Yep, i'm here to tell ya this 36 watt forum is about to get much busier. LOL! But hey, as slow as it is it could use some new life. My 4k heyboer OT just arrived. So i need to get busy and start ordering things. I'm building a 36 watter *basically*, but i will be using 6V6 outputs and a cascaded pre. So i'll start this new journey by asking a few questions..

1-who has a chassis with five octals, (4 6v6 and 1 rectifier) three 9 pins, and a cap can hole? If no one, how about a blank thats more than the 6.5" wide ones i've been finding? I found a wider one, but it's too long at 20" when i want no more than about 17". This will be a standard combo chassis, that is, front mounted controls like a marshall combo. (no slanted face like fender)

2-whats your opinion on the hammond 270HX PT? "176VA, sec. 275-0-275, DC ma 200, Fil.#1(rct) 5.0v @ 3a ct, Fil.#2(htr) 6.3v @ 6.0a ct." Theres a lower model that has 150ma, but thats not enough is it?

3-is the JJ 5AR4 good, or at $16 should i go elsewhere? Consider i have to order JJ outputs from Eurotubes anyways, and going elsewhere will mean shipping charges at 2 places. So unless theres a considerable difference or some reason to but a different rectifier i'd prefer to buy from Eurotubes.

[Alexo]

1) Hammond makes 17x10x2,3 or 4 chasses, available from Angela instruments. I'd have to recommend a steel one, though they are tough little f%$#&ers to work on, as the Hammond aluminums will be too light for the kind of transformers you'll be mounting.

2) I built an amp last month using that PT, running a pair of 7868's or 6V6's, switchable. It's a good PT and the power suplly keeps up with giant low-end settings really well with a SS rectifier, but the PT does get hot after an hour or so of running. I read somewhere that this is "normal" for Hammond PT's, and in my experience, it is. Just be aware of that, you also might want to parallel some .47 ohm 2 watt resistors and run them in series on one of your heater legs, as their heaters run a bit hot.

Running just one pair of 6V6's and using a solid state rectifier, I think my plate voltage is around 360, cathode voltage subtracted, yours would be a good deal lower - 330? 320? If I were building it, I'd run the 6V6's in fixed bias with a plate voltage of 425 or so. But different strokes for different folks.

Good luck!!

[lovedat800]

Running just one pair of 6V6's and using a solid state rectifier, I think my plate voltage is around 360, cathode voltage subtracted, yours would be a good deal lower - 330? 320? If I were building it, I'd run the 6V6's in fixed bias with a plate voltage of 425 or so. icon_twisted.gif But different strokes for different folks.

Well, realize that i know about .02% of what i probably should to be building amps. LOL! So i'm just going by what i know so far....what i've been told and what i see on the schematic. Therefore what i say i'm going to do is always questionable. So when you say that, could you elaborate on why you'd choose fixed bias and much more plate voltage? From my limited experience with the 18, i found that higher voltages seemed to make the sound harder while lower V seemed to soften it up and make it warmer. The latter being my tonal preference. So therefore i'd like to know your reasoning to see if i should maybe rethink this.

[Alexo]

Sure. But bear in mind that these are all my own personal preferences and may not translate to your ears or anybody else's.

First the fixed bias thing: To me, 6V6's are prone to a massive low end, it's one of their advantages and one of their disadvantages. So they are going to try and push this big low end through, and in cathode bias, this can result in a giant muddy fart. If you build in fixed bias with a strong power supply, the amp can keep up better with the 6V6's demands and you can actually have control over that big, thick, punchy low end. However, in the afore-mentioned build, I used ZP's 1,000uf cathode cap trick in cathode bias, and it seemed to work really well..

Voltages - 6V6's can handle higher voltages than EL84's, in general. 18 watts run the EL84's at pretty high voltages, a good 10 or 15% above maximum. This works out to be a relatively conservative plate voltage for 6V6's. But since you're not converting an EL84 amp, there's no reason to compromise. It's a matter of personal taste, but I think 6V6's really start to open up and sound big and dynamic at higher voltages (400+) and you'll definitely get more headroom, if that's what you're into. Just be sure to use JJ's or good NOS at those voltages. You can experiment with different rectifier tubes to raise or lower the plate voltage, but you'll affect sag as well.

[lovedat800]

Ok, then what OT would you recommend, or maybe i should ask, how do i get (or CAN i get) 400+ V from the 270HX?

[Alexo]

If you run fixed bias, you don't have to subtract the cathode voltage from the total voltage, so you can basically tack on an extra 25 volts to what I said before, so you could probably squeeze out 380 or so with an SS rectifier. It's kind of a guessing game though! A 272JX will push you up over 400 and give you a 250ma current rating, which I'd feel more comfortable with, especially if you dig the higher headroom.

Have you tried the Duncan Amps Power Supply Designer program? It's free and works pretty well, I'd run some simulations for you but I don't have access at the moment.

Haven't really tried 400+ in cathode bias, so I'm a little hesitant to recommend that....

[lovedat800]

Actually i was more interested in what you said about voltage but not fixed bias. I DO like headroom, tho then again four 6V6's should get me all i need in most any config i would think. And I'd much rather go CB for several reasons. I'll have to think about this and see what some others say....as you said it IS one man's personal preference.

[lovedat800]

Ok, forget that for now. Instead of starting another thread let me ask a few more questions. Because god knows if i do that for each question my name will be all the way down the topics list to the bottom. (this forum is slow....doesn't anyone build 36'ers?)

On the screens and the cathode, how should i do this and what values for four 6V6's? First, the cathode R i used on my 18 6V6 amp was 250k as Phil recommended. so should i use two 250k R's, one for each pair? And screens.....one R for each pair, or one R for each tube at the socket, and what values. I need to know asap because i can't order all my parts till i know exactly what i need.

Now someone best answer me or i swear to god i'll start another thread !

[JohnnyCrash]

I am completely with Alexo on this. Just as before, I'm sure you already know the tonal challenges of converting an 18w TMB to 6V6's - and then cascading throws a big wrench into the gears.

I just got finished with my 6V6 TMB cascaded 10 minutes ago...

First, I tend to prefer my cascade amps with big clean power sections. This means SS rectification, huge filtering, fixed bias, and negative feedback.

Second, you may find a good chassis for this from Ceriatone if you go with their "Plexi 100 Superlead."

As far as changes for 6V6's I've gotten a fair bit of mileage within the last 2 weeks experimenting like crazy.

A typical JCM800 has two treble peaking circuits in the preamp, I find I preferred only the first treble peaking circuit in the cascade section between V1's two triodes. I left off the second 500pF cap between V1 and V2. I also changed the treble cap in the TMB from 330pF to 500pF (already standard in the 36w). These changes left me miles of treble, but not as much high gain "fizz" (since I reduced some in the first stages).

I can get a great Fender 6G3 brown Deluxe clean with the Gain/Pre knob turned down. I tend to turn the treble down a lot for other uses, but I need that extra top end on-tap for clean stuff.

Second, I changed the PI to the Marshall Studio 15 4001's values. 470 ohm sandwiched by two 470k, with a 22k tail (mine is 27k though). The negative feedback attached here is pretty much lifted off of the 6G3 Fender brown Deluxe - as is the fixed bias circuit. It's a 56k off of the 8 ohm OT tap at the 0.1uF cap parallel with a 1.5k resistor.

In the tonestack I went with 0.01uF caps instead of the 0.022uF in the 18w/36w layouts. Instead of the 47k slope resistor I went with a 33k.

So far, my 6V6 18w doesn't sound woofy, has ACDC/Cult style grind, is giggabley loud, and has a fantastic clean sound... the Bass control is STILL only really usefull from 0 to 2.5 on the dial. Still trying to figure that out

Going with SS rectification, a 100uF filter, and a choke might be another option to get you tighter JCM800 style bass.

[lovedat800]

Well, i suppose i could try SS rectification and put a tube in later if i don't like it. I'm just trying to do this with as little change as possible because every thing i do different than the schematic will take me longer and longer to get around to actually doing this. The more changes, the more questions, the longer it takes till i can start. And it doesn't even guarantee me anything better than if i just stick to the 36 watt design with 6V6's. Every thing i think of changing leads to many more questions that need to be answered before i can start. I know because i've already had to put a lot of ideas aside because it takes too long just to get one thing answered. In fact, many questions get replies that lead to a even more confused state and more questions. So i'm trying to build the preamp i already know like the back of my hand and the 36 watt design with 6V6's.

So i think i just want to get some answers to the cathode R and screen R questions. then if the amp sucks i'll start changing things. But i think with a different PT and OT and all new parts it should be much more likely to be free of major tone issues like the 18. It could have been something weird like a bad tube socket, so just starting fresh has gotta give me a much better chance at success. I no longer really think it had anything to do with the preamp gain because thinking back it has the same issues as a stock TMB to some degree, and AS bad with the cascaded pre with a stage removed, which left it with a relatively mild amount of gain.

EDIT: i don't think this new build is going to be problematic like my 18 at all. I just tried tacking in the new OT and using the 4 ohm tap like Phil said to do, and plugging in the new 6V6 JJ's. The bass issue is so not there. The treble issue seems to be gone too, tho i will have to try it at volume before i can be sure on that. But man o man, the sound is nothing like it was. Huge and powerful. So anyways, i don't think i'm going to have to worry about adding NFB or fixed bias or ss rectifier. But i still need an answer to that question i asked earlier..........i was using a 250 ohm cathode R on the 18 with 6V6's....will i need to wire up TWO 250 ohm R's, one for each pair for the new amp with four 6V6's? Don't make me start a new thread now!!!