TC 8W Tube Amp Project

TomCat · 2009-07-10T18:13:17+00:00

Ok.....I decided to start a new thread following from Karel's thread about his 5W amp project. I've decided to go for a 8W Class A Amp head as a project. I...

TC 8W Tube Amp Project

TomCat

Alan Ratcliffe wrote: Damn, that's efficient for a 10".

The Emi 10" Ragin' Cajun has been getting a lot of rave reviews from guys upgrading their stock 10s in smaller amps like the Princeton. More of an American voice though. Both have good low end for a small amp.

If you really want the extra headroom, wouldn't it make more sense to build a 15W amp?

I thought about going 15W with dual OP tubes etc......I will probably go that route as a second build as a compact combo later.....add a reverb tank and all......

I like the idea of a single OP design for now as the build will be simpler....This being my first journey into the tube amp world....

Heath

have a look at the AX84 project on the net , seems to be heading in your direction . www.ax84.com , and the guys are extremely heelpful

Karel-Mars

Alan Ratcliffe wrote: How feasible is it to design a small amp with an output valve you can swap for others? Something like the Cornford Carrera which allows you to use any octal or nine pin power valve. It also has three preamp valves (single channel) which can be swapped with any combination of 12A_7 valves. Always thought that was nifty - the ultimate low powered 12" studio amp.

I looked at the Carrera and I think an ultimate low powered studio amp is worthwhile prototyping. I'll be tempted to leave the reverb and the fx loop out and use only two 12A_7's for single channel pre-amp with master volume... at least for starters.

The AX84 guys have a Single Ended Lead amp that seems to be the one called for when you want Marshall style tone controls.

TomCat

@ Matta and Karel.......PM'd Ver-1 of the schematic to you.

Simplified things somewhat in that it is now only single channel. Took out the OD channel for now.

In Summary:
> Single Clean Channel with single I/P
> Bass / Treble Tone Controls
> Volume Control
> 1 x 12AX7 Pre Tube
> 1 x EL34 Power Tube
> 1 x GZ34 / 5AR4 Rectifier tube

Looking to build as a small head with 8 & 16 ohm O/P.

Will keep you all posted on progress......

Gearhead

Sorry TomCat about the confusion 84/34.

If I can make some unsolicited advice: go for a different rectifier tube. Your EL34 and 12AX7 combined will eat 90-110mA while the GZ34 outputs 160. Compare the GZ34 to for instance the EZ40: The heater requirements of the GZ34 will consume twice as much energy and (since it is directly heated) will require a 5V of the power supply. The EZ, as the name suggests, can be heated from the same 6.3V as your other tubes which makes the amp simpler and the power tranny cheaper. It will also have more sag which will make for a spongier sound.
If you have any trouble sourcing EZ40 locally, I can quite possibly help.

For OPT I would go for the Hammond 125DSE. At 2500Ohm you will get 'only' 9% distortion on full input swing, is going to sound goooooood ?

TomCat

Hey Gearhead....No prob..... 8) 8)

Thanks for the advice on the rectifier tube.....definitely worth some serious consideration.

I want something that will sag nicely....as you say to get lekker spongy.....

Also thanks for the heads up on the O/P tranny as well....I presume Mr Valve will have that.

TomCat

@ Gearhead

Just had a look at the specs on the EZ40, which seems to be 90mA. I see there is also an EZ81 which is much beefier.....but maybe not desirable.....apparently very scarce.....

Just a question........Will the 90mA limit of the EZ40 contribute to the sag....and therefore the songy sound you are referring to?

Gearhead

The sag in any recto tube is a function of the output power. Some tubes sag more than others, so you will find a graph of the voltage versus current in most data sheets.
Now that I think about it: your amp will have no sag, being all class A. My mistake :-[ (although a common one). If you want sponginess, go class A/B or even B for which you will need push/pull.

TomCat

My main goal with the amp is to get reasonable clean gain at practice and mic'ed church stage levels.

Had a Peinceton on stage at church (an early one...Silverface) last week and seemed to "not quite make" it with clean gain on stage with a full band....there was a little....although not unpleasant amp breakup....

Based on my requirement, will the EZ40 still make it with the EL34 and the 12AX7 running off it....

Gearhead

If your EL34 will suck up 70 or even 100mA will depend on the type, the plate voltage and the biasing - I can't tell from here. You can tell yourself however - go get the data sheet of the specific tube you're using and work your way through the 'typical application' values or use a graph below. If I can do it, you can. If it is more than 90mA, well, go for the EZ81.
Getting to the right voltages is not so easy with tube rectification. I found the DuncanAmps.com psu designer very useful indeed, you can just put in the tube you use and it knows the performance.

TomCat

Thanks Gearhead

Will work through as you suggested. Thanks for the input.....much appreciated....

Karel-Mars

Here is the info for an EZ81: (copied from http://tdsl.duncanamps.com/show.php?des=EZ81)

EZ81 Ratings
Vh Ih
6.3 1.0

EZ81 Application Data
Class RMSin Irect Cout Rser
FW rectifier 350-0-350 150 50 230

The amp should sound great with an EZ81 (can deliver 150mA) rectifier. Even the first cap (47uF450V) is on target.

No shortage. JJ makes a 6CA4 with which the EZ81 is interchangeble/same. I am sure Mr Valve has stock. You can also get it on the 'net

http://search.store.yahoo.net/cgi-bin/nsearch?vwcatalog=yhst-8476489043850&et=4a67b7b0&vwentropy=1248211216&query=EZ81

Moving on to the EL34 (http://tdsl.duncanamps.com/show.php?des=EL34WXT

EL34WXT Ratings
Vh Ih VaMax Vg2Max VhkMax PaMax Pg2Max IkMax
6.3 1.5 800 500 100 25 8 150

EL34WXT Application Data
Class Va Vg2 Vg1 Ia Ig2 Ra S Zout Pout THD Notes
A S/E 250 265 -14.5 70 - 73 10 -15 18,000 9 3,000 8 10 With Rg2 = 2000 ohms
A S/E 250 265 -13.5 100 - 105 14.9 - 29.0 15,000 11 2,000 11 10

My guess the amp will consume about ~70mA at 380 B+

EZ81 is safe and probably a good choice.

AlanRatcliffe

My AC15 uses an EZ81 and I've bought from two different sources: Mr Valve and KCA NOS.

TomCat

Alan Ratcliffe wrote: My AC15 uses an EZ81 and I've bought from two different sources: Mr Valve and KCA NOS.

That's a fairly old AC15 then. Don't the newer ones have SS rectifiers?

AlanRatcliffe

Mine is the Heritage Hand-wired. Very nice in that it has both the EF86 and the Top Boost channels - all the classic Voxy tones in one amp. Based on the vintage designs, but some nice, modern tweaks (Triode and pentode switching on the O/P section and in the EF86 channel and better voicing on the "Bass Shift" switch and "Brilliance" switch). Here's the schem: http://ratcliffe.co.za/sundry/vox.gif

The CCs may have SS rectifiers, I'm not sure. But then, my '65 Twin is a SS rectifier and that sounds great.

AFAIK, if you design an EL84 amp with a SS recto, you'll need a standby switch. Another option for are Ted Weber's Copper Cap rectifiers - Nice! Faster "warm up", but still slow enough to be nice to the valves and obviate the need for a standby switch. Also need a lot less power from the PSU than a valve does.

Sean

Alan Ratcliffe wrote:
The CCs may have SS rectifiers, I'm not sure. But then, my '65 Twin is a SS rectifier and that sounds great.

Yes, it's the CCs that have the SS rectifiers.

TomCat

A Question regarding the O/P Transformer Primary Impedance.

The Hammond 125ESE transformer windings are as follows

I presume the primary impedance is determined by the load on the secondary.

i.e if the yellow wire is terminated to an 4ohm loudspeaker, the primary impedance is 2.5k. @ 8ohm it is 5K etc..its been a while since I did my transformer theory at college...... :-[

Manfred-Klose

Tomcat, found this website with a few handy calculators one can download:

http://www.duncanamps.com/software.html

aja

Looking at Manfred's link, wouldn't this be the kind of amp you should look at designing?

http://www.duncanamps.com/b112/b112.html

TomCat

@ Manfred...Thanks for the link......Really handy stuff there......appreciated.

@ Aja......Nice design....Push/Pull which is not what I'm looking for at the moment. Will be looking at a 15W P/P design later so will keep this in mind.....Thanks for the link anyway.

Back to the amp.....Design is basically done now so the next step will be getting the Power Tranny, O/P Tranny, Choke, Valves and bases so that I can put the chassis design together.

Once done I'll start posting pics etc......