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. In summary: Channel-1: 12AU7 Pre for the clean channel (Hopefully I'll get enough gain to drive the O/P Tube properly), I/P Gain Control Only Channel-2: 12AX7 Pre for the OD channel, Gain and Volume Control Output Tube: EL34, No Volume Control. Use an output attenuator - Full 8W, 4W, 1W ALA Vox AC4TV style. Loudspeaker impedance 8 / 16 ohm selectable if possible with an output attenuator. Rectifier Tube: GZ34 Overall Bass and Treble EQ No Reverb or Termelo The main goal for this project is a guitar amplifier head that gives both clean as well as OD tones. Let me know what you think and wish me luck.......

FenderBender

Wow Tom! Looking fantastic :applause:

matta

Wow!

Tom it looks great ? I'm amazed to see how over the past few months this Forum, category has grown from general amp info to guys like you, Geert and Mike building your own amps, it it great to watch!

I ordered your handles and corners yesterday, will be here in a few weeks and nice finishing touch to an already amazing looking amp!

Cheers

Matt

TomCat

Small update

I've had no luck with the Celestion blue I've wanted for my amp. However popped into Music Connection last Saturday and happened to ask them about any Celestion stock they might have. Turns out that the had some greenbacks in stock.

At first I was a bit apprehensive about the Greenback as I was not looking for the "Marshall" type of tone. Anyway....They were prepared to give me the loudspeaker on appro to try out, which I obliged.

I fitted the loudspeaker and tried it out.....

To make a long story short.....WOW...was I surprised.. ? ? ? ?.......No harshness or fizzy treble.... :woohoo:.....just a deep rich full tone (Almost too dark but responds well to a bit of bass cut).....very Fender like on the cleans. Sound especially nice on my ARC300 bridge position.

Switch to overdrive and the amp jumps into life......full deep tone with nice overtones...yet again not harsh at all......So for now the Blue is on hold and the Greenback stays......especially at about the third of the price of a Blue.

Edit: We tried the amp out at church on Sunday morning and it sounded very nice....easily kept up with the drummer on stage....I now have someone who wants me to build them a tube Bass Head.....Hmmmmm..... :-\ :-\

AlanRatcliffe

Nothing wrong with a Greenback (or a Vintage 30 for that matter), and they are fairly affordable (especially compared to a Blue).

I emailed Ron yesterday about Golds and apparently they are a special order at R4,595 ea. ☹

Manfred-Klose

I now have someone who wants me to build them a tube Bass Head.....Hmmmmm.....

yeah that stuff happens.

Gearhead

Saw a Gold second hand already. E175,- not so bad... :-\

AlanRatcliffe

Watford valves sells them for GBP155 new. Just waiting to hear about the shipping...

TomCat

Picked up a small problem by my own making.

I now have my corners etc to finish the amp but forgot to get screws at the same time.... ??? :-[

What I'm looking for is in the image below (the black one in the top left corner of the image).

Anybody got any ideas where these are available locally?

Edit: What I might do in the mean time is use short drywall screws. By no means ideal but they are black and the head hopefully won't protrude too much. I'll get some tomorrow and try it out.

Tamago

That is quite a common screw... my dad used to bring bucketloads of them home.

Were apparently used on Japanese motor vehicles he used to work on.
I can ask and see if he as any left, but yes, I remember them, we used them for everything.

TomCat

Tamago wrote That is quite a common screw... my dad used to bring bucketloads of them home.

Were apparently used on Japanese motor vehicles he used to work on.
I can ask and see if he as any left, but yes, I remember them, we used them for everything.

Fantastic. If you do have left over and are prepared to supply some to me I'll be extremely thankful.

Manfred-Klose

The screws in the top left corner are regular ones you get inside a truck that holds the dashboard together.
Maby try an automotive part supplier.

there is alot of bolt/nut suppliers in joburg.

Tamago

My Opel Corsa's dashboard is held on by identical looking screws come to think of it...
Except they are TORX heads ☹

TomCat

Amp has now been going for a while now and all is well.....really enjoying this amp.

A couple of things and a question or 2 for the tube guru's on the forum:

I've measured my DC current through the cathode bias resistor and it is sitting on 62mA (29.5V/470r), very close to the 64mA calculated.

The anode / cathode voltage is 402V - 29.5V (lets call it 30V) = 372V.
That tells me the anode power dissipation is 23W if I'm correct.

I want to reduce the value of the cathode resistor from 470R down to 390R, making the current through the 6CA7 approx. 75mA and the anode dissipation approx 28W. I see the 6CA7 seems to run at about 25W in most SE designs. Am I getting too close to the limit of the 6CA7 by upping the current from 62mA to 75mA with my plate voltage sitting at 402V.

My other option is to go for a 6550 which seems to be happier at 28W anode dissipation.

Gearhead

25W is the design center maximum rating, to exceed would be asking for trouble (red plating ain't funny). Can't you get a 430R? It's in the E24 series. Otherwise, two 820R in parallel? A 680R and a 1k?

Karel-Mars

Hi Tom

Why do you want to increase the current? Does it sound better at higher current?

Usually you tweak the 470R by paralleling something of appropriate value.

Regarding bias settings: Check this article:

http://www.aikenamps.com/Why70percent.html

A class A amp runs at max dissipation at idle. So 25W is the limit. But the important thing to check is if your screen grid is in the safe dissipation range. This part of the pentode is more vulnerable to damage (thin wire spiral) - and when it goes, your tube goes. By comparison your Anode (big plate) is tough.

Usually a 1K resistor is used to limit the current going to the screen.

TomCat

I'm curious as to what the effect would be if the current was increased....apart from more tube dissipation....How does it affect tone, would there be more power available through the OT etc.

I think at 23W I'm a little weary of going any higher and my screen resistor is indeed 1k so I'll not mess with that.

TomCat

FINALLY.....I have a clip from my amp..... :woohoo: :woohoo:

This clip is from recordings we are doing for Mariann's album.

Setup:

My Ibby ARC300 (Neck & Bridge) -> Proco Rat -> Amp (with cascaded gain switched in) -> AKG D3800 mic -> Tascam FW1880 interface.

Recorded on Reaper with no effects then added some delay and reverb as plug-ins.

Andre played the lead part......I'm not nearly that good..... ?. BTW the rhythm behind the lead is also Andre using the same setup.

http://www.box.net/shared/rz15icx3pg

AlanRatcliffe

Sounds great - Marshally without the harsh presence. Be nice to hear it just on the verge of breakup.

CDee

Sounds really good Tom, can't wait to hear it in person soon... with the Tele... ?

TomCat

Alan Ratcliffe wrote Sounds great - Marshally without the harsh presence. Be nice to hear it just on the verge of breakup.

+1 Alan.....That is exactly how I interpreted the sound......Clean is very Fender like but the OD channel is like a Marshall with nice warmth and overtones.....

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