Stoneham

Does the bottom of the amp have a removable panel? Or is everything accessed from the top? The mains toroid is the one under the black cover, there's a good chance it has additional primary taps to convert to 240V. It's a job for a tech though, there could be very high voltages inside there!

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

Measured at the onset of clipping, so maximum clean power

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.

No it's 17.5" wide, happy I can get this down to 16"

Hi everyone, after a very challenging year I'm now picking this project up again. A prototype (built onto a YI200 chassis) will be ready in next few working days. Been really struggling with the coffee maker attachment! The easiest thing would be to combine 2 x 6550/KT88 with a high voltage supply, say 650V. Most high power valve bass amps use this. It works, but high voltage is a valve killer. I've been working on increasing efficiency so I can use a regular 500V supply and prolong valve life. I've got a bit further to go but have just obtained 90 watts from 2 x KT88 with a 450V power supply which I'm excited about. This first prototype will put out about 90 watts, until I get a custom transformer set wound.

I've had an idea of changing the way the standby circuitry works - so while on standby, which removes high voltage to the power valves, there would still be power supplied to preamp and enable the DI out to function as well as rest of preamp. Then, if a valve pops during a gig (which normally stops the amp from working at all) you can at least get a DI signal out of it. Very minimal cost or complexity to do this

So far I'm going with a 3 band EQ, but will be capable of boost or cut and will be flat with dials at 12 o'clock. It should have the option of some pre-amp drive so a gain switch could offer that. Yes sliders add a lot of complexity to a small production run, plus pots are so much easier to service or replace long term. Speaker outputs - jack and speakon, 8 ohm and 4 ohm outputs. I don't like impedance switches due to the high currents, it's a weak link. Transformer driven D.I output. Do you guys bother with send/return jacks? Or a pre-amp output

Thanks to everyone for your input, enthusiasm and support on this! I'll be starting work with the transformers, as they dictate size and weight constraints and of course performance! The YI200 used a toroidal output transformer, but this had to be rated at over 300 watts to maintain 200w at 40Hz. I've learnt a lot out transformers and will be switching to traditional EI designs for output, and keep toroid for mains to reduce weight. It's still gonna be heavy but nowhere near SVT levels. I made a 100W valve bass amp for the bassist in my old pub band, had volume to spare and solid as a rock, which was the inspiration for the YI-200, should have just started with that but I wanted more power 😁

Hi I'm trying to track this down.... do you still have it please?

[quote name='jetofuj' timestamp='1395139409' post='2399016'] Rack mountable with optional head sleeve would be great! I'd personally keep the amp as simple as possible - especially that I'd like to see in the next few months, not years . DI is generally very useful, as soundguys are usually very lazy and prefer it. Is the amp actually based on Super Bass/JCM800? [/quote] My prototype has a unique preamp design that's not based on anything else, although it uses a passive tone circuit similar to a JCM800. I'm getting a lot of requests for a rackmount chassis, which I'm giving consideration; however I have some concerns about the practicality of this design. Even around 20 kilos, its a lot of tension on the four rack fixing points, most rackmount gear is solid state and relatively light weight so no problem, a big valve amp won't like being suspended horizontally and any knocks during transit would likely cause warping at the fixing points or even shear the bolts. Personally I would feel safer with a product enclosed in a heavy duty, ventilated cabinet, like an SVT but smaller (and lighter!), as long as it looks right on a cabinet. The other issue with rackmount is the heat that would be radiated upwards, not good for sensitive solid state preamps/effects. If you're a bit old school and just want an amp plus cab rig, a rackmount amp necessitates a rack case to house it in which is a bit pointless. Just thinking out loud here. How would you lot manage with a heavy, hot, rack power amp then?

I can do a lot with the preamp to shape the tone, so I intend to experiment with a valve drive circuit for dialling in extra grit at lower volumes. It could even be a 2nd channel, and footswitchable if needed. I'll be using local players as guinea pigs as I do prefer clean bass tones myself (I'm a lead guitarist hence why I'm relying on other peoples ears)

Hi everyone, Martin here from Stoneham Amps, thanks to everyone for your feedback so far this is very informative. To answer a few points raised, the prototype shown above was built out of curiosity and loaned to the bass player in my own band; he was that taken by it I realized I may be onto a winner so I'm taking the concept further. This amp has 4 x 6550, is it the general consensus that KT88 are the way to go, and what are your thoughts on 6550 themselves (I used them as I had some!!). Thanks to a compact aluminium chassis this weighs in at 20 kilos, so rather than a forklift, a couple of weetabix in the morning should be all you need. The criticism of the width overhanging the cab was expected and duly received, the final product shall be standard 19" width. I'll be looking at offering various power options, probably just 200w and 400w to start with. 200w does sound low to some but I would really recommend listening to a 200w valve amp before ruling it out