B15-inspired Amp Head Build

[JapanAxe]

18W can be massively loud for a valve guitar amp. I gig a 15W Matchless and it has never been ‘not loud enough’. As has often been discussed here, bass amps generally require a bit more.

No you can’t just plug headphones into the speaker output. There are solutions involving a load box but it can become a bit pointless.

Yes you can connect one of these heads to a bass cab of the appropriate impedance, and you may be able to optimise the amp for bass by changing some of the components e.g. coupling capacitors.

Vyse Amps do a 120W bass amp kit. Both Vyse and Modulus do a Bassman kit.

[JapanAxe]

Bits and pieces have been arriving, including parts of the enclosure. Here are the loosely assembled chassis and cover shown next to my Demeter head for scale. There will be plenty of room to work inside the chassis, which is always nice!

[JapanAxe]

All valves now acquired:

2x Sylvania 6SL7GT, JAN standard

2xShuguang 6L6GC, current production - test as well-matched on my Orange valve tester

1x JJ GZ34

Some other parts also delivered today!

[Passinwind]

19 hours ago, JapanAxe said:

All valves now acquired:

2x Sylvania 6SL7GT, JAN standard

2xShuguang 6L6GC, current production - test as well-matched on my Orange valve tester

1x JJ GZ34

Some other parts also delivered today!

Nice! When I was doing my first rack mount tube preamp I was able to take home several 6SL7 variants from the local repair joint my friend owns to try in the built up preamp and just buy the ones I really liked. All of them were good and it came down to what I thought the guy I was building it for would like best. Lots of different looks here:

 

I ended up picking the second RCA from the left for that build and the brown base Sylvania as a spare for my own earlier build. Used as the front end in my circuit the  RCA was/is a little warmer and "tubier" for lows and low mids and the Sylvanias are subjectively a bit more "etched" or lively in the upper mids and treble. Best of all for my taste was these other Sylvanias though, which have a very tight feel and no microphonics issues at all, unlike a whole lot of old 6SL7s I've tried:

 

And of course in a B-15 circuit or some other application I might very easily prefer something else.

 

 

Edited April 23, 2021 by Passinwind

[JapanAxe]

3 hours ago, Passinwind said:

Nice! When I was doing my first rack mount tube preamp I was able to take home several 6SL7 variants from the local repair joint my friend owns to try in the built up preamp and just buy the ones I really liked. All of them were good and it came down to what I thought the guy I was building it for would like best. Lots of different looks here:

 

I ended up picking the second RCA from the left for that build and the brown base Sylvania as a spare for my own earlier build. Used as the front end in my circuit the  RCA was/is a little warmer and "tubier" for lows and low mids and the Sylvanias are subjectively a bit more "etched" or lively in the upper mids and treble. Best of all for my taste was these other Sylvanias though, which have a very tight feel and no microphonics issues at all, unlike a whole lot of old 6SL7s I've tried:

 

And of course in a B-15 circuit or some other application I might very easily prefer something else.

 

 

6SL7s are a new venture for me. I’ve always built with 12A*7 preamp valves (tubes) before. The 6SL7s I’ve bought are guaranteed, and I am building a head rather than a combo this time so microphonics would be less likely to be an issue.

The fact that they are octal valves is a major plus as it is much easier to make solder connections to the sockets!

[JapanAxe]

I have started on a full-size layout for this build. As per my plan, the Ceriatone Aunt Peg uses just one preamp channel but provides the choice between cathode biasing (models up to 1964) and grid biasing (1968 models onwards). I have blown up the board layout to match the 70mm width of the phenolic board.

To fit both cathode bias and grid bias components I would need to split the board into two sections. The PT I have ordered doesn't have a dedicated grid bias tap, but I could use the bias supply circuit from one of the later models that runs off the full HT. Alternatively I may just stick with cathode bias - this would be the more 'vintage' option, I don't need 5 extra watts at home, there would be less to get wrong, and I could easily fit everything on a single board. I would have a few components spare but there will always be another build!

I am also tweaking the circuit in other ways, for example I will have just one input socket with a 33k grid stopper and 1M grid leak resistor, and I will be using a multiple star ground scheme as advocated by Merlin Blencowe.

I have the dimensions for the transformers but I don't yet know where the leads will exit, so I won't go too far with the layout until I am sure of this.

 

[Bassybert]

Watching this thread with much interest as this is something I’ve thought about doing for a long time!

 Have toyed with the idea of making a rack preamp version of this circuit but the power section is so key to the sound making a preamp would be like having fish without chips, Cannon without Ball and Dempsey without Makepeace. 

[JapanAxe]

I probably won't get the transformers for another week as they are being built to order. In the meantime I have been considering my options as regards the heater filaments.

The Ceriatone layout follows the Heritage schematic in having the heater centre tap connected to ground. I would normally do this, or in the absence of a centre tap on the PT I create a virtual CT using 100R resistors between each side of the 6.3V AC supply and ground. Both the 1964 and 1968 schematic show a 100R Hum Balance ('humdinger') pot across the 6.3V, with the wiper (marked K) connected to the cathodes of the output valves, which will be at about 36V above ground at idle. As I understand it, elevated filament voltages are intended to distance the heaters from any noise in the circuit ground, and the pot allows any residual noise on the 6.3V lines to be cancelled out. With proper grounding there shouldn't be any noise, but I think I will compare connecting the heater CT to the cathodes or to ground to see which produces the lower noise floor. If neither approach is satisfactory, I can consider adding a hum balance pot.

If anyone has personal experience of heater noise in the B15 circuit, please do share!

[JapanAxe]

I have been carefully considering the grounding scheme for this build. I have rearranged the layout to employ a gound bus as described in Chapter 15 of Merlin Blencowe's book on guitar and bass preamps. All ground connections are made to the bus, which connects to the centre tap of the HT on the left of the drawing and to the input socket at the right. That is where it will also be connected to the chassis to set it at ground potential. The ground return for each amplification stage is made immediately after the ground connection of that stage's reservoir cap. The speaker sockets will not be grounded at the rear panel but next to the ground of C19, which is the reservoir cap for the phase inverter, as the NFB resistor feeds the PI grids.

I have also moved the tone stack onto a separate board, which will be connected to the ground bus next to the reservoir cap for the first preamp valve. No other connections are shown - once the transformers arrive I will crack on with the full layout.

Edited May 8, 2021 by JapanAxe

[JapanAxe]

I tried to rearrange the central section of the layout to eliminate under-board connecting wires, but the new layout didn't really work so I ended up re-drawing the earlier layout. This time I added the volume control, which I may move to the other side of the tone stack.

Now that I am happy with the board layouts, I have marked out the positions of holes for eyelets (dots), mounting screws (circles), and through-holes (diamonds).

Edited May 10, 2021 by JapanAxe

[Passinwind]

On 01/05/2021 at 14:23, JapanAxe said:

I probably won't get the transformers for another week as they are being built to order. In the meantime I have been considering my options as regards the heater filaments.

The Ceriatone layout follows the Heritage schematic in having the heater centre tap connected to ground. I would normally do this, or in the absence of a centre tap on the PT I create a virtual CT using 100R resistors between each side of the 6.3V AC supply and ground. Both the 1964 and 1968 schematic show a 100R Hum Balance ('humdinger') pot across the 6.3V, with the wiper (marked K) connected to the cathodes of the output valves, which will be at about 36V above ground at idle. As I understand it, elevated filament voltages are intended to distance the heaters from any noise in the circuit ground, and the pot allows any residual noise on the 6.3V lines to be cancelled out. With proper grounding there shouldn't be any noise, but I think I will compare connecting the heater CT to the cathodes or to ground to see which produces the lower noise floor. If neither approach is satisfactory, I can consider adding a hum balance pot.

If anyone has personal experience of heater noise in the B15 circuit, please do share!

 

I pretty much always just use DC heaters for preamp level circuits these days. It's been a long time since I owned my B-15, but I dont remember it being nearly as quiet as the stuff I've built myself over the last decade or so. That amp was always a bit of a problem child though, unfortunately.

It's very cool seeing a  hand drawn layout, I've been CAD-only for several years now. 😉

 

[JapanAxe]

52 minutes ago, Passinwind said:

I pretty much always just use DC heaters for preamp level circuits these days. It's been a long time since I owned my B-15, but I dont remember it being nearly as quiet as the stuff I've built myself over the last decade or so. That amp was always a bit of a problem child though, unfortunately.

It's very cool seeing a  hand drawn layout, I've been CAD-only for several years now. 😉

 

I’ve read that DC heaters can bring their own issues. Heater noise hasn’t been a problem in my other builds, and I can just try the options and see what works best.

On all but my first build (which was a kit) I have drawn a full-size chassis layout so as to be sure that everything will fit without any parts fouling.

I have chased the transformers...

[JapanAxe]

Drilled the boards and set the eyelets:

 

Edited May 17, 2021 by JapanAxe

[JapanAxe]

Ground bus added

[JapanAxe]

The iron is here! It might be a while before I get much done though, as currently a medical condition is making it impossible for me to sit at a desk or workbench for more than a few minutes at a time.

[JapanAxe]

When I say ‘medical condition’ it’s nothing you can die from, just a pain in the ar$e - literally!

[Bassybert]

Sciatica?

[JapanAxe]

2 minutes ago, Bassybert said:

Sciatica?

No, ischial tendonitis. Trying not to aggravate it at the moment, and unfortunately sitting in an office chair does just that! Weirdly, cycling is fine. Also I can still play guitar or bass if I sit on a high stool and find the right position.

[Chienmortbb]

Slightly off topic. I don't remember where I saw it but recently someone was restoring an old valve amp and intended to replace the transformers. Having made transformers at EMI in the late 60s, I don't think manufacturing methods or  materials have improved to the point that there is any point in changing the "iron".  Or have I missed something?

[Beer of the Bass]

1 minute ago, Chienmortbb said:

Slightly off topic. I don't remember where I saw it but recently someone was restoring an old valve amp and intended to replace the transformers. Having made transformers at EMI in the late 60s, I don't think manufacturing methods or  materials have improved to the point that there is any point in changing the "iron".  Or have I missed something?

Yeah, I wouldn't replace them in an older amp unless they were dead and a replacement was cheaper than a re-wind. They're such a fundamental component that it seems a bit Ship of Theseus!

[Chienmortbb]

I must say that those two shiny transformers look so good although my back is twinging already.

[Chienmortbb]

On 15/05/2021 at 16:32, JapanAxe said:

I’ve read that DC heaters can bring their own issues. Heater noise hasn’t been a problem in my other builds, and I can just try the options and see what works best.

On all but my first build (which was a kit) I have drawn a full-size chassis layout so as to be sure that everything will fit without any parts fouling.

I have chased the transformers...

The two main issues are, Hum with AC heaters and some extra heat with DC heater supplies. On balance I think I would go DC as I would not want 50/60Hz anywhere near my amplifying devices.

[JapanAxe]

23 minutes ago, Chienmortbb said:

Slightly off topic. I don't remember where I saw it but recently someone was restoring an old valve amp and intended to replace the transformers. Having made transformers at EMI in the late 60s, I don't think manufacturing methods or  materials have improved to the point that there is any point in changing the "iron".  Or have I missed something?

On previous builds I have re-used and recycled ‘old iron’, not least to save on cost. This is a new build from scratch and after researching the topic I decided to go for these transformers from Primary Windings.

14 minutes ago, Chienmortbb said:

The two main issues are, Hum with AC heaters and some extra heat with DC heater supplies. On balance I think I would go DC as I would not want 50/60Hz anywhere near my amplifying devices.

I am currently reading Merlin Blencowe’s excellent book and have just read the section on heater supplies. He suggests running just the first stage heaters on DC as DC doubles the current requirement compared to AC.

I have never had problems with noisy heaters on previous builds (5 valve amps and a 6G15-style standalone spring reverb). If it turns out to be a problem I have various options - humdinger, elevated heater voltage, and DC as a last resort!

28 minutes ago, Chienmortbb said:

I must say that those two shiny transformers look so good although my back is twinging already.

Yeah it was a fairly weighty package! Fortunately the head will just sit on top of my BF One 10 in my music room and I won’t have to schlepp it around too much...

[JapanAxe]

The tendonitis has improved to the point where I can sit and work at a desk, so I have finished the layout today. I have had to carefully work out the positions of the transformers so that none of the mounting bolts and grommet holes conflict with the board stand-offs - you can see my paper mock-ups to the right of the photo. With the PT position I have chosen, the primary wires would come through the chassis right next to the preamp, but I will run them the other side of the chassis and bring them in at a different point (yet to be decided) to prevent induced noise and feedback. I haven't shown the 6.3V heater wires

I will have a fresh look at this tomorrow before committing myself to making holes in expensive metalwork!

[JapanAxe]

I realised I had the board stand-offs coming out under the transformers, which wouldn't work. In the second draft I moved the transformers a bit closer to overcome this, but then I wondered whether the PT might induce hum in the OT, so I set them up as below. The PT primary is connected to the mains and the screen to the earth. All the unused PT wires are parked safely in chocolate block connectors.

The OT is mounted orthogonally (look it up!) to the PT and I have my headphones connected across the OT's 16 ohm tap. I found that the transformers really need to be at opposite ends of the chassis, and their relative position is crucial to minimise induced hum. When it comes to it I will mount the PT, then move the OT around on the chassis before marking up the bolt positions. Rather than mount the eyelet board with 2 stand-offs at each end, I will settle for one at each end at the front and one in the middle at the back. The board is 3.2mm phenolic and very rigid so I am happy this will be sufficient.

Measure twice, cut once? No - measure, check, measure, check, sleep on it, measure, check...