my 18 watt rocket-ish build

[rock_mumbles]

Sounds like it's not bias correctly.

Good point Mark!

What size is your EL84 cathode resistor?

[M Fowler]

OP said 150R and cathode was 11.74.

[rock_mumbles]

Thanks ... I looked but couldn't find it

[wsaraceni]

What's the way to calculate bias? I'll take the measurements tonight.

[rock_mumbles]

What's the way to calculate bias? I'll take the measurements tonight.

From the voltages you've posted earlier the dissipation should be in the ball park ...

You can estimate the bias with the Weber Bias Calculator use the third one ... you need the plate voltage, cathode voltage and cathode resistor value.

To calculate the actual dissipation you need the screen current also ... the voltage drop across the screen resistors and their value ... you subtract the screen current from the cathode current to get the actual plate current from that you get the dissipation in watts ... an EL84 is a 12watt dissipation tube

[wsaraceni]

thanks. i think someone helped me with picking the 150R when i first put it together. i think at this point i will try the additional 1k just going to the power tubes, leaving the rest of the amp alone. i have a few 1k 5w ohmite resistors (part number 45F1K0E) that i can try.

[wsaraceni]

bumping my own ancient thread.


I built this amp. it looks good. it sounds ok-ish. nothing great. I built this before there was a z-wreck jr. and based it off of what I believed to be well researched info.


so now I'd like to finish it and get it sounding good. going off of the c-wreck CeriaTone layout, im very close to the same component choice with a few major exceptions. mostly being the 2 x el84s only, the ez81 rectifier, and the non-master volume.


I started with 18 watt marshall transformers and a choke. im not sure if those could be part of the reason I dont like the sound.
here are those part numbers

mercury tone clone mc10h choke
HTS-7046 OT - 18 watts - 9960948
18 w PT-290 HTS-10194 9961024

I believe these were all for a marshall 18 watt from metropolous amps.


im going to document over the next few days where I differ from the CeriaTone c-wreck and hopefully someone here can help me adjust.

starting with what I can see in the power supply. im using 40uf 500v sprague atom and 8uf 450v instead of the 47/47/10/10/10 in the c-wreck

[wsaraceni]

https://www.freestompboxes.org/viewtopic.php?t=19924

found this. my original voltages match up pretty good to this for the B+ readings

[wsaraceni]

how much does the primary impedance of the OT matter. im wondering if that might be my main problem.

[Fischerman]

I recently built a Matchless Lightning clone (which is in that same family and around 15W) and also an 18W Liverpool.

The Lightning used alleged Matchless transformers (from amppartsdirect) and the OT is 3k7 ohms at 4/8/16 ohm secondary which is around what a 4xEL84 amp would be. It’s pretty bass heavy when the impedance selector is set to match the load but I run it as a 8/16/32 ohm so it’s more like 7k4 ohms.

In the Liverpuddle the OT is a custom MM Fender Deluxe OT that is around 5k9 ohms at 2/4/8 ohm secondary. That's close to the same primary impedance for a 4xEL84 Liverpool. But I run that amp as 4/8/16 ohm so it’s more like a 11k8 ohm primary.

Both amps sound great and with plenty of fullness/muscle/girth.

Here’s what Samon from Mercury Magnetics wrote when he suggested the Deluxe OT.

Here’s what I’ve come up with for a 2x EL84 equivalent to the Stancor 3801. To recap, I figure you’d need something with around 10-12k primary impedance, good for 20 watts or so, and with an interleave pattern that would match the sonics of the Stancor.

Like mentioned, it’s uncommon to find an OT with a designed primary impedance higher than 10k. One main reason is that once you get that many turns of wire, you start dealing with a lot of inter-winding capacitance (behaving like a cap shunting the plates) which really starts to dull out the tone.

The solution is usually to find an OT with half the primary impedance you’re after, then mis-match the secondary impedances so that the load reflects over as double. We have a F-DELUXE-FS-OM ($234 retail, $211 your cost) that is 5.9k : 2, 4, 8. If you treat the secondary as a 4, 8, 16 instead, the load would reflect over to the primary as a 11.8k. Check out the wiring diagram attached that shows the connections.

[wsaraceni]

this might be a stupid question, but with everything unplugged should my heaters and output jack have continuity with ground?

[martin manning]

OT secondary has one end grounded and has very low resistance, heater winding is also very low resistance and has a grounded center tap, so yes.

[wsaraceni]

OT secondary has one end grounded and has very low resistance, heater winding is also very low resistance and has a grounded center tap, so yes.

thanks. in that case I think I built a very functional, but very boring sounding amp.