JPF 100W valve bass amp prototype.

jpfamps

Here are some snaps of the prototype of a 100W guitar/bass head.

The initial motivation for this amp is that I wanted my own bass amp. At the moment I am using a friend's Bassman 100, and I expect at some stage hoe will want it back.......

I wanted to try to produce an all valve 100W bass head in a relatively compact package, that will have something of an SVT vibe.

Although this amp was initially designed as a bass amp, we've had some interest from a couple of guitar players, so there will be a guitar version as well.

The amp fits in our "standard" headshell, and will weigh in at around 14 kg.

For the more technically minded here are some of the amps "interesting" features:

1. Same treble/bass control topology as our other amps
   
2. Defeatable active mid cut/ boost using a tapped inductor a la Ampeg, which gives 5 selecteble frequencies to cut and boost.
   
3. Fixed bias long tail pair phase inverter driven from a DC coupled cathode follower
   
4. 2 x KT88 giving 100W output power
   
5. Unlike our other amps we've employed negative feedback around the power amp, so we can have a conventional presence control
   
6. Toroidal mains transformer to reduce hum and save weight (this will be potted in a production model)
   

We've got a few issues to sort out with this amp. The primary loading is a bit on the low side, we used a 100W transformer that we had knocking about (this often  happens when prototyping....)

The HT is a bit higher than I would like (740 VDC, 370 on the screens), which is OK for KT88 but too much for 6550s, and I would like to have the option of using either valve in this amp, so we will need to adjust the windings of the mains transformer. Ideally I would like around 680 on the anode and around 380 on the screens.

Anyhow, here are the photos:

The front (sans control panel). We're aiming to use some different cosmetics on this amp, but I wanted to weigh it in a head shell. We'll also incorporate a vent in the top of the headshell.

The back. Bias test points, and outputs for 4, 8 and 16 ohms. This is what the transformer we used had, but this can easily be user defined.

Mid control inductor. Wound by Sowter. Note the mu-metal can for low hum.

Finally some gut shots, and yes Dave there is no insulation on the mains inlet/ switch (yawn).

ddlooping

Very cool.

ThePrettyDamned

Looks pretty sweet...

Mind if I ask, what is the main difference between a bass valve amp and a guitar valve amp?  I've plugged a guitar into many a bass amp head, and they've always sounded great (all valve bass amp head I mean).

Love the KT88's, proper massive! 

ICBM

Neat .

Given the efficiency of a lot of modern lightweight bass cabs this should be enough for gig volume even for more modern bass tones than I remember you saying you use.

I'd avoid putting a beer trap in the top of the cab though - not necessary and it's, well… a beer trap! (And I know you shouldn't put drinks on amps, especially bass amps.) A slot at the top and bottom of the back panel to cause a good convection path is all it needs - old Orange 120s only had that and they don't overheat.

Are the plastic valve bases OK at 740V? I know in theory the dielectric is plenty, but having seen a few arced and carbonised ones, even at lower voltage… although admittedly mostly on Marshall DSL/TSLs.

jpfamps

ThePrettyDamned said:

Looks pretty sweet...

Mind if I ask, what is the main difference between a bass valve amp and a guitar valve amp?  I've plugged a guitar into many a bass amp head, and they've always sounded great (all valve bass amp head I mean).

Love the KT88's, proper massive! 

Good question.

Without trying to raise the hackles of any bass players on thsi forum, my opinion is that there is little difference.

Certainly back in the day there were just amps, some of which had a normal and bass pre-amp. (Incidentally I use the "normal" and not the bass channel on my friend's Bassman 100 and I still have to turn the bass down).

Then it all went wrong.........

Our other guitar heads sound great for bass, just not that loud, although our 30W head has been gigged for bass.

Fine for recording though. In fact valve guitar amps are often used for recording bass.

You do need to employ different cabs though. 

If there are any difference between the "guitar" and "bass" versions of this amp it will be that the bass amp may have a larger OT (extends bass response and will have less insertion loss), which may not have a 16 ohm tap (although of course this could be specified by the end user). We may also try trimming the gain of the 1st stage back a bit as bass guitars put out more signal than guitars, and you want to avoid clattering the 1st stage.

ThePrettyDamned

Neat, thanks mate.  Looking forward to some demos

jpfamps

ICBM said:

Neat .

Given the efficiency of a lot of modern lightweight bass cabs this should be enough for gig volume even for more modern bass tones than I remember you saying you use.

I'd avoid putting a beer trap in the top of the cab though - not necessary and it's, well… a beer trap! (And I know you shouldn't put drinks on amps, especially bass amps.) A slot at the top and bottom of the back panel to cause a good convection path is all it needs - old Orange 120s only had that and they don't overheat.

Are the plastic valve bases OK at 740V? I know in theory the dielectric is plenty, but having seen a few arced and carbonised ones, even at lower voltage… although admittedly mostly on Marshall DSL/TSLs.

Modern bass sounds, like the 70s! 

JJ Burnell rocks.

I know your aversion to the "beer trap", and I will give it some thought.

The puritan in my would probably want to make the amp more reliable in normal use, at the expense of vulnerability to rank stupidity. 

I've seen several OR120s with scorching on the back panel. Mind you they do run the valve ver-r-r-ry hot.

This amp is more compact than the OR120, although I don't run the valves quite as hot.

The back will have a metal grill, so maybe small vents in the top will suffice.

The Belton sockets are glass impregnated and are fine at these voltages, which are a bit higher than I wanted.

Technically ceramic sockets are better than these sockets, but I don't like them as I've seen quite a few amps with cracked sockets. Furthermore if they are not fully glazed (and the ones everyone uses aren't they can absorb moisture, eg contact cleaner.

An external arc can char the Belton sockets, necessitating a change of socket, however  even with ceramic sockets there can be enough carbon deposited on the socket to require it being changed (especially if it's unglazed when cleaning only seems to grind the carbon into the ceramic).

I find the Belton sockets are easier to mount with a spring retainer than the standard ceramic sockets, and the pins are easy to retention. Having said that we did use ceramic sockets in our earlier amps (mainly it must be  said because we had a large stock of them, and they fitted the existing metal work).

Mind you I quite like the look of these PTFE sockets:

http://www.ebay.com/itm/Hi-End-Grade-TEFLON-8-PIN-TUBE-SOCKET-x-1pc-/111199009528

martinw

jpfamps said:

ICBM said:

Neat
I'd avoid putting a beer trap in the top of the cab though - not necessary and it's, well… a beer trap! (And I know you shouldn't put drinks on amps, especially bass amps.) A slot at the top and bottom of the back panel to cause a good convection path is all it needs - old Orange 120s only had that and they don't overheat.

 

I know your aversion to the "beer trap", and I will give it some thought.

The puritan in my would probably want to make the amp more reliable in normal use, at the expense of vulnerability to rank stupidity. 

I've seen several OR120s with scorching on the back panel. Mind you they do run the valve ver-r-r-ry hot.

This amp is more compact than the OR120, although I don't run the valves quite as hot.

The back will have a metal grill, so maybe small vents in the top will suffice.

 

 

Neat indeed.

I've had similar thoughts about top vents. As an engineer, I want them, definitely. There's a phenomenal amount of heat billowing out of them on some amps, which clearly would convect more slowly given less efficient ventilation, leading to an overall temperature rise.

As somebody who builds amps for use in the real world, I'm going on trust more than I'd like when I put them in!

On balance I'm happier with them there on anything above about 20W.

I've had no trouble in the 7 years I've been doing this, hopefully due to marginally more careful owners

martinw

jpfamps said:

Finally some gut shots, and yes Dave there is no insulation on the mains inlet/ switch (yawn).

Lol!

ICBM

martinw said:

Neat indeed.

I've had similar thoughts about top vents. As an engineer, I want them, definitely. There's a phenomenal amount of heat billowing out of them on some amps, which clearly would convect more slowly given less efficient ventilation, leading to an overall temperature rise.

As somebody who builds amps for use in the real world, I'm going on trust more than I'd like when I put them in!

On balance I'm happier with them there on anything above about 20W.

I've had no trouble in the 7 years I've been doing this, hopefully due to marginally more careful owners

Maybe I just get the dozy owners . It's not that uncommon to find an amp seriously damaged by a drink in through the top vent. I genuinely don't like them or think they're necessary - there are a lot of amps which don't have them (most combos for a start) and which don't overheat.

I think it's more important to create a convection path - but you must arrange a cold air inlet as well as a hot air outlet, otherwise it won't work properly. If you do, you need a hot air outlet *at* the top but not at all necessarily *in* the top, if you see what I mean . A vent at the top and bottom of the back panel is possibly more effective even than a full-height grille, in fact - it will produce more of a continuous flow once the air gets warm.

I don't like fans either, but if the amp needs that much cooling it's probably better to fit one. I've had a lot of success with deliberately slow-running them - the easiest way is to use a 240V fan but connect it to the 120V tap of the PT (if the amp has one) - the fan turns slowly and silently, but even a little help is enough to get the convection working well.

martinw

ICBM said:

I think it's more important to create a convection path - but you must arrange a cold air inlet as well as a hot air outlet, otherwise it won't work properly. If you do, you need a hot air outlet *at* the top but not at all necessarily *in* the top, if you see what I mean . A vent at the top and bottom of the back panel is possibly more effective even than a full-height grille, in fact - it will produce more of a continuous flow once the air gets warm.

I know, and agree It's difficult to get 2 separate vents in an attractive way when you factor in the manufacturing techniques available to a small builder. The way I do it is by spacing the rear panel of the cab backwards by 5mm, leaving a small gap between panel and chassis at the bottom to go with the top mesh vent. It works well and is cosmetically undetectable.

You don't really need 2 vents if the single vent is big enough. I'm sure you know that the cold and warm air streams will laminate through a single vent, and if it's big enough it will be efficient enough to provide adequate cooling.

I don't like fans either, but if the amp needs that much cooling it's probably better to fit one. I've had a lot of success with deliberately slow-running them - the easiest way is to use a 240V fan but connect it to the 120V tap of the PT (if the amp has one) - the fan turns slowly and silently, but even a little help is enough to get the convection working well.

I do this with Power Scaling installations where a fan is required, using a 12V fan off the 7-8V you get from the heater supply. This moves air slowly over a heatsink, and the difference with and without is startling.

GilgaFrank

Interesting idea with the AF inductor there, don't recall seeing that done on a modern production amp. Sowter, verrrrry expensive aren't they?

jpfamps

GilgaFrank said:

Interesting idea with the AF inductor there, don't recall seeing that done on a modern production amp. Sowter, verrrrry expensive aren't they?

Inductors were widely used in equalizers probably up to around the early 70s, when they were largely replaced by gyrator circuits.

Ampeg implemented a variable mid/cut boost using a tapped inductor on the SVT in the 60s, and still use a similar circuit.

Fender used inductors in the Super Bassman eq, and MESA also used them in the graphic eq on their amps.

Inductors are expensive though, and we thought that the mu-metal screen can was a good idea to reduce hum. This added to the cost as well.

EdGrip

Love your work.