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100W Stoneham valve amp - small footprint, lighter weight

Merton

By Merton,
in Amps and Cabs

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Would you be interested in a small-run, small footprint 100W valve amp from Stoneham?  

59 members have voted

  1. 1. Would you be interested in a small-run, small footprint 100W valve amp from Stoneham?

    • Yes
      50
    • No
      9

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Matt P Experienced

Matt P

Posted
Posted

If i remember rightly somewhere in this thread Martin said that he would be using a traditional transformer for the output and a toroidal for the mains as a balance between weight and cost, 

 

as a slight bonus a pair of KT88's is slightly cheaper than 4 6L6's (and i'm guessing easier to package in the smaller space)

Matt

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northcountrybob Community Regular

northcountrybob

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You'd be really pushed to get 100W out of a pair of EL34s. I've come across an old Bogner (iirc) doing about 90W on two EL34's, and there are other vintage amps and run EL34 that hard, but I wouldn't think it's a good idea if you want a reliable modern amp. The KT88 is just designed for higher dissipation. More robust screen grids. 

 

One problem with running high plate voltages like 600+ is that it's not uncommon to get arcing across the octal valve bases from plate to filament pins. You'll find in AD200B's that sometimes from the factory they have a little heat shrink ring around the tops of the plate pin or filament pins on the 6550's. Could be an idea to run DC elevated heaters. I also have a vague memory of Pin 1 on some KT88's like the JJ's maybe being tied to the metal valve base??? Just means during the build that may need considering if you want to avoid any voltages ending up on the exposed metallic part of the valve. 

 

The JPF is pretty small. Standard OT, Toroidal PT, Sowter inductor mounted on the top. You might get it small by having a custom taller / slimmer toroid, I think the space saving would be negligible. Probably don't want everything crammed together and cooking either. If you're going for a fully metal enclosure it may well get toasty. Side handles may be a shout! 

 

 

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Stoneham Explorer

Stoneham

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For the tech minded, and anyone interested, I'm restricting HT to about 500V to counter the problems mentioned above.

It's still possible to achieve 100W with 2 x KT88 or 6550, I've already obtained 90W with a 450V supply on test.  

I plan on having a bias switch so EL34 can be used - will drop to say 80W but that should still be enough, and gives more options for user.

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Chienmortbb Grand Master

Chienmortbb

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The decision whether to choose an EI laminated transformer and a Toroid is not a simple one. I remember working on a TV monitor that was designed for an EI transformer. It was decided to change to a toroid to reduce weight and cut costs. All went great until switch on and HUMMMMMMMMMM. The picture had horizontal bars running up/down the screen. The magnetic field of a toroid is much tighter and well-defined than an EI, BUT in this case the stray magnetic filed was focused on the CRT neck and causing a brightness difference. The number of hum bars depended on how far the mains frequency was from 50Hz as TV timebases were very accurate but mains varies in frequency.

 

In the right position the toroid can reduce magnetically induced hum in am amp but in the wrong place? I am sure that @stoneham is aware of this though.

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Stoneham Explorer

Stoneham

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Posted

I get all my transformers, toroid and EI, custom made to my spec. Have used toroid mains in guitar amps for a few years now, I find they perform better for stray fields.  

In practise, output toroids can work great but need to be oversized to handle sub 50Hz frequencies properly,  a big deal on a bass amp.  I've learnt loads from the YI200 about transformer design.

All ABM amps have a huge mains toroid, likewise Trace Elliott and many others.  It's a case of finding the right part for the job

 

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  • 2 months later...
JPJ Mentor

JPJ

Posted
Posted
On 20/08/2023 at 21:55, Stoneham said:

I get all my transformers, toroid and EI, custom made to my spec. Have used toroid mains in guitar amps for a few years now, I find they perform better for stray fields.  

In practise, output toroids can work great but need to be oversized to handle sub 50Hz frequencies properly,  a big deal on a bass amp.  I've learnt loads from the YI200 about transformer design.

All ABM amps have a huge mains toroid, likewise Trace Elliott and many others.  It's a case of finding the right part for the job

 

Did somebody say huge toroid?

IMG_0785.jpeg

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