Wouldn't it be great if you could build lots of amps with just one set of transformers? There would be a big cost saving if you could re-use the most expensive components.
The idea for this project began when I couldn't decide which 15-watter to build next: Matchless Spitfire? Lightning? Goldtone GA15? 5E3 Deluxe? Maybe the Divided by 13 CJ11, a modern take on the Tweed Deluxe (the same cathodyne PI power amp with an AB763 preamp)?
All of these amps can (I think...) use the same pair of Hammond 372BX and 1615A transformers so I've decided to try a modular design. A "hub" unit will contain any circuit elements which can be shared: trannies, a dual EL84 power amp, a dual 6V6 power amp, and the power supply up to the second filter cap. Separate boxes will contain preamps and also part of the power supply.
Time I showed you some schematics. These are all for the current version 0.3:
the "hub" unit
Spitfire/Lightning preamp/PS
Goldtone GA15 preamp/PS
5E3 preamp/PS
CJ11 preamp/PS
Here's a draft layout for the hub unit. There may be the odd wiring error so don't try building it.
proposed hub layout
So far I haven't built anything which didn't come as a kit with instructions so I'd like to ask a few questions.
(1) Do you see any obvious problems with the layout? The PT and OT are at the back on opposite sides of the chassis with the choke roughly in the middle. I've got bits of circuit quite close to them and sometimes directly underneath. I've no idea how much space I need to leave around them to avoid picking up hum.
The other decision which dictated the shape of the layout was to put the tubes reasonably close to the front panel volume controls. This avoids long wire runs to the front panel and back but it also puts the heater wires down the middle of the chassis.
Apart from that, I just tried to arrange things to avoid wires crossing or running parallel as much as possible and where wires do cross, make them cross at right angles.
As soon as I get some confirmation that I haven't done anything obviously stupid I can start drilling out the chassis.
(2) Not entirely sure if I've got the circuit correct around the hub schematic switch S5b. Is it grounded correctly? Got to get that right...
(3) Also in the hub schematic, C1 and C21 are the first and second filter caps - except that there is no single cap but rather several in parallel with a bunch switches so I can provide range of different values. There is a resistor at the end of the array intended to allow the caps to drain. There are also resistors connected across the switches, again to allow the caps to drain. Does 220k sound right for these? Higher?
Also, AFAIK, there are limits for the size of the first filter cap which a given tube rectifier can handle (something to do with the ripple current with a big cap). All the caps switched into the circuit together would be too much for a 5AR4 for example. However, when they are switched out we still have the drainage resistors "shorting" out the switches... Is this an issue? I'd guess that the large size of the resistors means the tube rec doesn't get zapped with massive ripple current but I don't know for sure.
Thanks for your help - any other comments please let me know 🙂
The idea for this project began when I couldn't decide which 15-watter to build next: Matchless Spitfire? Lightning? Goldtone GA15? 5E3 Deluxe? Maybe the Divided by 13 CJ11, a modern take on the Tweed Deluxe (the same cathodyne PI power amp with an AB763 preamp)?
All of these amps can (I think...) use the same pair of Hammond 372BX and 1615A transformers so I've decided to try a modular design. A "hub" unit will contain any circuit elements which can be shared: trannies, a dual EL84 power amp, a dual 6V6 power amp, and the power supply up to the second filter cap. Separate boxes will contain preamps and also part of the power supply.
Time I showed you some schematics. These are all for the current version 0.3:
the "hub" unit
Spitfire/Lightning preamp/PS
Goldtone GA15 preamp/PS
5E3 preamp/PS
CJ11 preamp/PS
Here's a draft layout for the hub unit. There may be the odd wiring error so don't try building it.
proposed hub layout
So far I haven't built anything which didn't come as a kit with instructions so I'd like to ask a few questions.
(1) Do you see any obvious problems with the layout? The PT and OT are at the back on opposite sides of the chassis with the choke roughly in the middle. I've got bits of circuit quite close to them and sometimes directly underneath. I've no idea how much space I need to leave around them to avoid picking up hum.
The other decision which dictated the shape of the layout was to put the tubes reasonably close to the front panel volume controls. This avoids long wire runs to the front panel and back but it also puts the heater wires down the middle of the chassis.
Apart from that, I just tried to arrange things to avoid wires crossing or running parallel as much as possible and where wires do cross, make them cross at right angles.
As soon as I get some confirmation that I haven't done anything obviously stupid I can start drilling out the chassis.
(2) Not entirely sure if I've got the circuit correct around the hub schematic switch S5b. Is it grounded correctly? Got to get that right...
(3) Also in the hub schematic, C1 and C21 are the first and second filter caps - except that there is no single cap but rather several in parallel with a bunch switches so I can provide range of different values. There is a resistor at the end of the array intended to allow the caps to drain. There are also resistors connected across the switches, again to allow the caps to drain. Does 220k sound right for these? Higher?
Also, AFAIK, there are limits for the size of the first filter cap which a given tube rectifier can handle (something to do with the ripple current with a big cap). All the caps switched into the circuit together would be too much for a 5AR4 for example. However, when they are switched out we still have the drainage resistors "shorting" out the switches... Is this an issue? I'd guess that the large size of the resistors means the tube rec doesn't get zapped with massive ripple current but I don't know for sure.
Thanks for your help - any other comments please let me know 🙂
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This is an interesting project! But for easy of operation and safety, I think a more traditional layout may be better - PT & choke on one end of the chassis, and the OPT on the other end, with the outptut tubes in the middle. Most of the switches can be located on the front panel (instead of on top of the chassis), so the hands and fingers do not get anywhere near the tubes for safety. All the I/O connectors can be located on the back panel. There are going to be perhaps more "crossed lines" than your current layout, but as long as you separate the high current ones from the low current ones, it should still be ok, just my 2c...
Fascinating project!
A 5AR4 is only supposed to feed a maximum of 60uF (and a 5Y3 only 20uF!). How about leaving the smallest cap in each of the banks C1 and C21 always on (without a switch)? The other two caps in each bank could be switched, but keeping the maximum total of C1 at 60uF? I think the range of reservoir and smoothing capacitance values for the different amp models will be interesting, but I don't think you need to keep to the exact values for the various amps in question.
I think 220k is a good value for the drain resistor (not sure if you need two of them though) but I would go up to a 2W resistor or higher. (The 1W is within its rating but it will get hot.) The drain resistors staying in place across the switches is not a problem.
Safety grounding and transferring a common signal ground (while avoiding loops) across the different pre-amps and 'the hub' needs some careful thought.
The circuit around S5B does not look right (to me) at the moment. The tube rectifier has to be a 'two phase' full-wave rectifier using the PT centre-tap. When you switch over to solid state rectification you need it to do the same. At present you are trying to switch over to a full-wave bridge (which will give you too much voltage).
A 5AR4 is only supposed to feed a maximum of 60uF (and a 5Y3 only 20uF!). How about leaving the smallest cap in each of the banks C1 and C21 always on (without a switch)? The other two caps in each bank could be switched, but keeping the maximum total of C1 at 60uF? I think the range of reservoir and smoothing capacitance values for the different amp models will be interesting, but I don't think you need to keep to the exact values for the various amps in question.
I think 220k is a good value for the drain resistor (not sure if you need two of them though) but I would go up to a 2W resistor or higher. (The 1W is within its rating but it will get hot.) The drain resistors staying in place across the switches is not a problem.
Safety grounding and transferring a common signal ground (while avoiding loops) across the different pre-amps and 'the hub' needs some careful thought.
The circuit around S5B does not look right (to me) at the moment. The tube rectifier has to be a 'two phase' full-wave rectifier using the PT centre-tap. When you switch over to solid state rectification you need it to do the same. At present you are trying to switch over to a full-wave bridge (which will give you too much voltage).
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Thanks. I didn't realise the choke would be OK sitting next to the PT.
The top panel switches are not to be used while the amp is switched on - mostly they're not even rated for live switching. You'd set up for a given circuit, switch the amp on, and after that they wouldn't be touched.
If different types of controls are grouped in different places it's easier to remember which you're allowed to use iefront panel controls (master volume) only when the amp is switched on and the top panel controls only while the amp is switched off.
I see, so it's more like a breadboard - no live switching, then I guess it's fine. Good luck with the project and let us know how it turns out.
I'm somewhere in the terrain between a full-blown prototyping rig and a standard tube amp. I want to create a viable, usable studio amp which can easily create a variety of dual EL84 and 6V6 circuits. There are more switches than you'd probably want to have in a gigging amp but (hopefully) not so many that it's difficult to use in a recording set up.
Just to be clear are you confirming that the resistors across the switches will create the same ripple problem as if all three caps were connected simultaneously (79u)?
For the amps I'm planning to support, I need 16u (5E3), 20u (CJ11), 33u (Spitfire/Lightning) and 100u (GA15). 22u should be close enough for the 5E3 and CJ11 without (AFAIK) significantly altering the character of the amp. Same with the maximum 79u for the GA15.
Thanks - I eventually chose 2W metal film as my default for all resistors, unless a higher rating is needed. I should update the scematic. Higher-rated resistors are quieter and 2W MF's are so cheap there's no point going lower.
Yes indeed! 🙂 I've got a six pole connector (Universal mate n lok) which will pass ground and B+ to pre-amps. Although I never intend to plug/unplug this connection hot, I will use special grounding pins (they make first contact before the power pin so you can't get zapped).
I was planning to pass two separate grounds: one for the power supply and one for the preamp tubes. I've got some spare pins if I need more. Overall, I'm trying to stay reasonably close to a star ground system. Each chassis will be connected to mains earth at one point.
Thanks - important info. I'm not entirely sure what I'm doing here. I was trying to merge schematics for the Goldtone GA15 and Matchless Spitfire. I should read up some more on rectifiers.
I've half a mind just to dump the ss rec. I'm not sure there would be any audible difference with a low-sag tube like the 5AR4. The only real reason to have one is in case I need the extra (10-20?) volts one day.
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The 220k resistor directly across the switch means that capacitor will still charge up when the switch is open, but very much slower than when the switch is closed. So, for example, if you have the switch closed on the 10uF cap and open on the other two, the ripple will be approximately the same as you would get from just the 10uF by itself.
If you want to keep the solid state rectifier option, just think of the valve rectifier as being like two diodes and switch over to replace it with 'two SS diodes'. In practice you might want to make up each of the 'two SS diodes' as a series string of two diodes with small parallel caps, to give a higher voltage rating.
If you want to keep the solid state rectifier option, just think of the valve rectifier as being like two diodes and switch over to replace it with 'two SS diodes'. In practice you might want to make up each of the 'two SS diodes' as a series string of two diodes with small parallel caps, to give a higher voltage rating.
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