Supro Dual Tone 1624T/6424 in a recycled Crate Vintage 30

[SpeedTrip]

Just starting to plan out my latest amp build. It will be my third, and the first one where I'm having to think the design through so open to any guidance along the way.

The last two were a Stinger Kit (Guitar Magazine Kit about 20 odd years ago) and last year a Modulus Amplifier's 20 Watt Marshbar (documented on 18 watt forum).

As prep I've acquired an output transformer from a 15 Watt Laney (driven by two push pull EL84s) and the cab, power transformer and chassis from a Crate Vintage 30.
I've posted the schematic here as a reference.

Id like to go down the route of the 6v6 version of the 1624T but I'm unsure if the voltage will be two low so I may have to use EL84s as documented by Sluckey elsewhere

So far

Output transformer (no centre tap) produces 270volts AC, 366 Volts DC when going through a bridge rectifier (its powering 4 preamp and four power tubes so out put should be fine).  Going down the route of a rectifier tube would drop this down to the 300V mark.

The other two ac outputs (both with centre taps) are a 24 volts AC feed for the circuitry (which wont be much use unless I come up with a solid state reverb circuit for the Reverb tank in the combo's base) and a 4.4v ac volt feed that is used with an artificial centre tap to provide both 3.6 volts AC and DC to power the heaters (measured without tubes in place). So the only tube rectifier that seems to fit from my reading would be an EZ81 (6.3 volts and can carry 150mamps).


 

[SEL49]

You'll have to use a hybrid FWB if you want to use a tube. I would not bother, but if you must, it will have to look like this...

     https://sluckeyamps.com/misc/VAC-4.pdf

[SpeedTrip]

Thanks for the input. Yes I'm aware id need to fit the two diodes to provide the "centre tap".  The two questions I have about using the EZ81 are around the voltage multiplier (I've see figure quoted of 1.1 to 1.3 dependant on mamps through the valve) and if this drops the voltage too much for 6v6s..and the second is more towards your comment - why bother as i'm not sure this valve sags the same as a 5y3.

[SpeedTrip]

Update on tube heater voltages. The AC measurement i gave of 4.4volts is incorrect, i found the pin id used into the terminal was slightly tarnished so i didnt have the right reading. Its still 3.6volts on the board (both ac and then dc after its gone through diodes but its 7.2 ac out from the transformer (Crate show 8volts on the schematic)  I plugged in the valves and the board figures drop to 3.4 volts ac and dc . So its seems Crate are running their power tubes at 7.2 volts ac and the preamps at 7.2 volts dc.  Given tube life ill need to find a way of cutting the volts down. Not sure if its easier to do all valves with ac or mirror Crate circuit but run all valves on dc (given Im going to have to put some resistors in the circuit somewhere. Sorry for previous spurious entry.

[Willabe]

1st off, measure ACROSS the tube heaters, not 1 probe to 1 heater pin and the other probe to ground with the meter.

Measure with 1 prob to each tube heater pin. 

Post what you get.

2nd, and tubes heaters can be run at +/- 10% acv/dcv. So, 6.3 x .1 = .63  and 6.3 + .63 + 6.93. and 6.3 - .63 = 5.67. So anywhere between 5.67 to 6.93 acv/dcv is fine.

Check to see if your wall acv is high. That could cause the readings to be high.   

[SpeedTrip]

Will do and Ill report back

[SpeedTrip]

Wall voltage 241 volts ac   (schematic shows transformer rated for 230volts ac)

Across power tube pins 6.66volts ac

Across preamp tubes 6.66volts dc

[SpeedTrip]

I also measured the heater voltage on the Modulus amp I built and its got 6.3 volts ac on same supply. But from your comments it looks like I'm ok to simply keep the heater output as it is and run ac to all tubes.

Thanks for input on tube specs.

[Willabe]

If the your donor amp PT ran the tubes acv and dcv, you can run them the same, if you use the same tubes and number of tubes, acv on the power tubes and dcv on the preamp tubes if you want.

[SpeedTrip]

Thanks for coming back on the heater stuff Willabe. Its an option worth thinking about as the chassis isn't that deep so dc lines may help to avoid hum. I'm scraping the existing board so I could possibly lift the diodes and resistors and build onto a small board away from the main circuitry (mind you for the price of new probably not worth it).

I've also looked through a few of the schematics of the 6v6 option now and from material looks as though if I remain with the bridge rectifier my B+ voltage will be within the right area for the power tubes.  Ill post a schematic when I've thought everything through.

[Willabe]

It's not about the depth of the chassis.

DCV heaters are usually used in very high gain preamps and very low signal preamps for microphones.

I use heater elevation, only 2 R's and a cap, with a humdinger, just 1 pot. Schematic below shows elevation on bottom left, humdinger bottom right.

Read this on dcv heater elevation and humdinger; 

 https://www.valvewizard.co.uk/heater.html

[SpeedTrip]

An interesting  read.  My comment on the chassis height was more about my build skill and fitting the wiring in but as you rightly point out that's not connected to heater circuit selection.

If I've digested the material correctly there are a number of solutions available with added circuitry involved.

For the amp I'm looking to build (not high gain) the simplest approach would seem to be leave it a standard ac supply, if i get hum then I've the  option would be to try a humdinger then leading on from that to the dcv heater elevation (and humdinger).

[Willabe]

For the amp I'm looking to build (not high gain) the simplest approach would seem to be leave it a standard ac supply, if i get hum then I've the option would be to try a humdinger then leading on from that to the dcv heater elevation (and humdinger).

Heater DCV elevation and a humdinger address 2 different sources of hum. That's why I used them together in the amp schematic I posted.

On a new build very easy to put in. DCV elevation is 2 resistors and 1 cap, the humdinger is 1 pot if your PT's heater secondary has a CT. If no heater CT, then you need 2 more resistors. Still very easy.

Edit; typo crossed out above.

The "ac leakage through the heater-cathode" trobbins is talking about is from a bad tube. Tubes aren't made as well as they used to be, so this helps you to be able to use an other wise good tube. 

Here, from trobbins;

"The humdinger pot may help with capacitive coupling from heater voltage over to input stage grids, as it can help neutralise the effect of such coupling (but could be worthwhile doing a temporary check by powering the heater wires going to the preamp circuitry with a 6V battery to see if that is a major hum contributor).  In contrast, the elevated dc voltage for the heater may help suppress any ac leakage through the heater-cathode interface and into the input stage cathode circuit (but worthwhile also tube-rolling to see if that can help lower the hum, as not all tubes have the same leakage)."

"Parasitic capacitance from each heater wire/side may not be balanced, such that some leakage occurs - with more leakage into a grid from one heater side than the other - that can be re-balanced by the humdinger pot.  However the heater voltage is not a pure sinewave, so some leakage frequencies may still leak into the grid, especially if the mains has significant distortion or flat-topping, and if the rectifier's operation is distorting the heater voltage (eg. likely worse with ss rectifier).

In contrast, a heater ct or fixed humdinger may still allow some unbalanced leakage (eg. checked by using a temporary 6V battery to feed the wiring into the preamp section)."

Those quotes by trobbins are from this thread, replies #10 and #12 ; https://el34world.com/Forum/index.php?topic=32255.msg356716#msg356716 

[SpeedTrip]

I'll have a think about the heaters and draw up how I think the elevated and humdinger would work.

Here is my working schematic. I used Sluckey's as this was easiest to mod and does seem to form the basis of many builds on this site. I did wonder about the 2.2kw resister shown on the original schematics between A and B (B+) but concluded the voltage loss would be too high so kept it at 1K.

[Willabe]

You don't have to use elevated heaters and a humdinger.

I just showed you those as an option to the dc heaters.

[SpeedTrip]

Fully appreciate that its an option, but given this amp building helps to keep the brain active Id like to at least see if I can draw up a circuit

[SpeedTrip]

I had a think overnight and looking at my schematic I think I'e another option. I'm using fixed bias at the power valves (not as high dc voltage as you option).  Sketch attached.

I do like the idea of simply adding something in at the build stage

[Willabe]

Yes you can do that.

But, if you look at my schematic, I take the dcv from the very end of the power supply. So that's taken from the very end of the eyelet/turret board and I inject it into the very 1st preamp tube.

So there's more room in the tube socket lugs for the 2 extra wires, so 2 pair. If you inject the dcv stand off in a different tube socket, now you have 3 pairs of wires.

Also the dcv is cleanest/smoothest there. I've heard some say that doesn't matter but it sure can't hurt. 

And I put the humdinger pot at the very end of the chassis too. It works out very well this way for layout on the eyelet/turret board and chassis.

[SpeedTrip]

Yep would work well as i can put the two resistors (and Cap) at the end of the board and the pot (linear I assume) out through the rear of the chassis. Thanks for persevering with me. Ill have a go at that as I loose nothing in trying it out.

[Willabe]

... and the pot (linear I assume)

Yes, linear.

[SpeedTrip]

Its a 380k resistor on the other side of the voltage divider for the B+ at the end of the run. Parts added to my pick list. Cheers

[Willabe]

Its a 380k resistor on the other side of the voltage divider for the B+ at the end of the run. Parts added to my pick list.

How do you know that it's a 380K?

You wont know until the amp is built and powered up. Then you use gator clip leads to swap in different value R's until you get the dcv you want.   

[SpeedTrip]

Just using the 380k as the starting point.

[SpeedTrip]

Ill post a few photos of the Crate now Ive started to pull it apart.

I've a good 4BB3C1B solid state reverb tank, so a combination of the Crate circuit to get +/-15volts DC and a reverb circuit from a Laney L5T lionheart using 072 op amps is a possibility at some stage.

I've got 3off 12ax7 preamp tubes that are fine, along with 4 off EL84s

Printed circuit board which I have no use for (I might hold in reserve just in case I can use any bits). If anyone in the uk needs the board the PM me. Just pay postage.

I've an output Transformer which was driven by 4 off EL84. Again if anyone in the UK has a use for the transformer PM me.
Photos to be added later from my phone

[SpeedTrip]

Photos now sized to allow posting

[SpeedTrip]

other photos

[SpeedTrip]

Board I dont need.  Edit 13/07/25   Ive worked out how the solida state reverb works so board has now been stripped. Im going to use this plus the 15 to 10volt power supply on a tag board. More later.

[SpeedTrip]

Output Transformer that is geared up for 4 EL84s.

[SpeedTrip]

I've actually started on the build. No time when you're retired .

Wired up the speaker to allow me to plug into other amps and also wire this amp with an attenuator. Ill draw up the circuit when Ive finished it but its based on Rob Robinette's designs.

I've had to mod the chassis to allow me to fit the 6v6s and other tubes in but Im happy with the arrangements. Ill modify  the photos and add them later

[SpeedTrip]

Photos

[SpeedTrip]

photos

[SpeedTrip]

and cab

[SpeedTrip]

Progressing well.

Ive got most of the components in and the transformer wired up and running .

Some figure - with the tubes in (3 at .3 amps and 2 at .5 amps) its showing 7.07volts AC. So ill need 0.15 ohm resistor (3watt wirewound for safety) in each of the ac legs to get the voltage down to within 10% over 6.3 volts.

- B+ voltage from the transformer is 275 and 278 DC after the bridge rectifier - there are no cap in yet so I'm hoping this will lift the DC volts but IM beginning to wonder if my reading of the 1.4 multiplier is correct. I may end up with artificial "sag"  (or buy a different transformer)

Secondary control circuit is showing 11.5 volts - and + on either side of the bridge , but again no capacitors are in the circuit yet.  Ill only develop a tag board layout if i decide to keep the transformer.

[SpeedTrip]

I thought Id test out the 1.4 multiplier on the dc voltage when you connect up a capacitor to the dc lines after the bridge rectifier. 
I robbed all the low voltage control circuitry from the Crate and put the components on a tag board.
From the 11.5v after the bridge I got 16.25 volts - and + and then after the voltage reduction got the controlled Zenner diode voltage of 10.1 volts - and +.  So I can potentially build the reverb unit as I have the voltage driver for it.

So I'm a bit more hopeful.

[SpeedTrip]

Wiring up progressing. I added an attenuator to the circuit on the outside of the steelwork. The chassis sits vertical in the cab so heat loss should be fine (- I can always add a fan!). Schematic next to resistors on the photo,

[SpeedTrip]

Up and running though Ive got to investigate a hum that is a little annoying.

I fitted a three way selector switch in the circuit to allow channel 1 and 2 to operate - independently, combined and then with channel 1 (after the volume pot) switching in to channel 2.  Means the volume pot on channel one acts as a gain pot.  Sounds a little top end as you end up using channel 2s pot only. Its with this arrangement that the hum gets annoying.  Shot below..More than welcome to have any thoughts. My immediate thoughts are probably more tightly wind the heater cables but the frequencies don't seem to fit this issue.

[SpeedTrip]

Some figure.

Voltages 353 volts dc at A, 345v at B and 284v at C

I used a 230k ohm pot on the voltage divide with the 100k to get 80volts dc in the heater circuit.

I also installed 0.15rohm resistor in the heater circuit to drop the voltage to 6.3volts ac.

You see in photo I fitted the low volt power supply as im going to progress with the solid state reverb, channel switch and FX cicuits at a later stage.

[SpeedTrip]

I used JJ 6v6s for the push pull and this needed the 250 ohm (255.2r actual) for the cathode resistor ( giving me 22 volts across across it).
v4 to centre was 118.4ohm 337.9 plate with plate to cathode of 316
v5 to centre was 107.5ohm 337.6 plate with plate to cathode of 318 volts

I used Rob Robinetts calculator for cathode bias and i was getting a range from the calcs of 12.4watts (92.1%) to 13.7watts (97.6%) so i decide that was fine

[tubeswell]

Wiring up progressing. I added an attenuator to the circuit on the outside of the steelwork. The chassis sits vertical in the cab so heat loss should be fine (- I can always add a fan!). Schematic next to resistors on the photo,

I’ve had mixed results with those resistors- they’re not always reliable. Maybe better to use 25W wire-wound ceramics for a 1/2 power attenuator. Look at the Fender EC series Tremolux as an example.

[SpeedTrip]

Thanks for the advice tubeswell. Ill keep an eye on them, Ive got them also in a effects box that I use for other amps.  So any issues Ill swap out.

Noise --- Got to the bottom of the slight annoying sound shown in the screenshot earlier.

I decided the waves looked to be roughly in the 50hz cycle (ie 50, 100, 150, 200 etc) so wondered about ripple from the capacitors.  Id followed my last build and used one earth for power and preamp section. But this had stronger first filter.

So I ran through normal checks with swapping out valves and checking circuit for ground loops and then rewired the earths for separate power and preamps. Problem gone. So I can now think about  the solid state stuff.

Final comment at this stage the three way selector works well, the two stage one after another drive the amp harder, I guess towards the Colorado design.

Oh and while I think about it - a big thank you to Modulus Amplification based in Scotland UK who were again helpful with parts.