What shall I build?

knuckleberryfinn

I've just been given an early 70s HH Ike Isaacs amp. The original (and nice sounding) Fane speakers are still in it, but the chassis is missing. So what shall I build to put in its place? I want a loud, rich, clean and simple amp for jazz duties.

I already have a bassman head, a Super Reverb, a 4x10 5e3 with blackface preamp, and a Champ, so something different from these. Maybe a 2 x EL34 thing with a Champ or Harvard pre amp? The speakers are rated at 100W together, and the cab is bomb-proof.

Winny_Pooh

Jtm45 with kt66 & big iron

knuckleberryfinn

A bit too close to the bassman probably @Winny_Pooh, but you've given me an idea. What about a single ended KT66, 88 or 120 Champ? I know Laney do a higher power single ended amp, but not with a single power valve. Anyone have any experience with a big single ended amp?

DannyP

I have some transformers that I'm looking to get rid of - a Hammond 290DEX and a Hammond 1650P

http://www.mouser.co.uk/Search/m_ProductDetail.aspx?Hammond-Manufacturing/290DEX/&qs=itkJorDwoMLUjRRU2U+iqQ==

http://www.mouser.co.uk/Search/m_ProductDetail.aspx?Hammond-Manufacturing/1650P/&qs=sGAEpiMZZMv0IfuNuy2LUcyh5J80v/VukRtke9/AdqU=

Let me know if they're of any use to you.

knuckleberryfinn

Cheers @DannyP. Please PM me prices on these.

knuckleberryfinn

I can't find any single ended OTs of more than 30 watts anywhere. Is there a way of getting round this? Can I use OTs in parallel to increase power handling?

fretmeister

Clean channel from a Twin

Crunch from a JTM45

Filth from a Dual Rec with a TS already in front of it.

And if it could weigh under 15lb that would be ace!

jpfamps

knuckleberryfinn said:

I can't find any single ended OTs of more than 30 watts anywhere. Is there a way of getting round this? Can I use OTs in parallel to increase power handling?

The Hammond "audiophile" 1642 SE OT is rated at 75W.

It weighs 13kg. That's 4 kg more than one of our 25W heads.

Other transformers in this range are rated at 30W and only weigh in at 5kg.

However, these transformer are rated down to 20 Hz, which is an octave below the low E on a bass guitar. Low E on a 6 string is around 80 Hz.

For extended bass response you need a much larger OT, so for guitar, where you don't need the extended bass, a 30W SE "audiophile" transformer will be easily good enough for 60W, with the caveat that you don't exceed the maximum standing DC current.

BowynMadley

With a kt120 in a combo you'll have to watch out for tube rattle. They are huge bottles! If you can't find an OT transformer bigger than 30w make it cathode bias. It'll sound better and should come in at about 24 ish watts

RiftAmps

Those big bottle valves require a lot of juice just to switch them on, let alone to run them at a reasonable operating point. It's very inefficient to do so and there are cheaper ways to get more power (PP anyone?)

Modulus_Amps

what do you mean higher powered single ended amp? more than one power valve?

A 15w - 20w OT would be more than enough for a single 6L6, EL34, 6550. I use this on a number of builds that use a single valve, it is over rated and handles all "standard" valves easily and makes a very giggable amp
You can use multiple OT for multiple valves if you want.
http://modulusamplification.com/15-Watt-SE-Output-Transformer-P5295332.aspx

The Hammond 125ESE is what allot of guys were using 8 years ago when I was big into SE amps

I do agree with RiftAmps though, if you want more than 10w then PP is the way to go.

knuckleberryfinn

Thanks @jpfamps, I had no idea I could do that.  The 1640SEA is optimal for parallel 6550s or EL34s according to the Hammond data, but would this be for cathode or fixed bias operation?  I'll have to have a good look at the data sheets and Merlin's books for DC current and all that when the kids bugger off back to school.  

As for push-pull, I've got three home-built PP amps already.  I'm just looking at building something a bit different, and thought a big SE amp would be an interesting beast.  

BowynMadley

My amp is push pull using two kt66s in cathode bias. Even so it kicks out about 28 watts on full tilt. I believe it has a 40w OT transformer in it.

RiftAmps

If you're going to do it, you might as well go full bore and run a KT150

jpfamps

knuckleberryfinn said:

Thanks @jpfamps, I had no idea I could do that.  The 1640SEA is optimal for parallel 6550s or EL34s according to the Hammond data, but would this be for cathode or fixed bias operation?  I'll have to have a good look at the data sheets and Merlin's books for DC current and all that when the kids bugger off back to school.  

As for push-pull, I've got three home-built PP amps already.  I'm just looking at building something a bit different, and thought a big SE amp would be an interesting beast.  

You can use either fixed or cathode biasing (or even a mix of both) for SE although cathode bias is usual.

The main reason why cathode bias is employed with SE is that they are usually biased to maximum power dissipation (or even above) to optimize power output, with the assumption that they are operating in class A.

In fact many SE amps aren't operating in class A, but never mind.

With cathode bias the valve will settle on its own operating point which will be around maximum dissipation regardless of the valve's characteristics, whereas with fixed bias the bias would need adjusting to ensure that the valve is not running way too hot as you have little margin for error unlike say a push pull class AB amp.

Having said that a fixed bias SE amp would be a unusual machine and I'm sure would sound great. If you are the only person likely to use the amp then you needn't worry about pilot error.

All OTs have a maximum DC current rating that it's unwise to exceed. In a push-pull transformer your sole concern here is damaging the transformer, however in a SE transformer you are also concerned about preventing the transformer saturating which will limit the signal and cause distortion. 

In a PP OT the transformer is wound so that the magnetic fields generated by the DC current through the two halves of the OT cancel out (and why you need a PI to drive the two sides the OT with signals 180 deg out of phase) , whereas with a SE transformer the DC current means there is a standing magnetic field in the transformer before you apply signal.

If there is too much current through the transformer then the transformer will saturate on positive signal excursion.