I'm building an amp with big bottles.

ezietsman · 2015-10-09T11:11:33+00:00

? I don't have all the parts yet so this thread will not be moving that quickly yet. I also have to finish my planning and design. I'm taking a classic 1960s design and I'm adding in the ability to switch its configuration from a 1966 spec to a 1968 spec. Here's a teaser pic in the meantime. Some of you will be able to tell what I'm going to do with the given information, who knows their amps?

V8

ez wrote I got as far as this today, maybe I can fire it up in the morning ?

The flyleads off the board are not soldered to the chassis yet, I'll do that in the morning, that's all thats still needed.

Looking super neat, nice work ez!

Fingers crossed for the play testing ?

ezietsman

V8 wrote Looking super neat, nice work ez!

Fingers crossed for the play testing ?

Thanks a lot ?

Many events today though, Star Wars and dogwalks and dinners (with booze, no ampy ampy) so it looks unlikely that today will be both Star Wars and 68 Plexi day ? Maybe, would be nice.

V8

ez wrote so it looks unlikely that today will be both Star Wars and 68 Plexi day ? Maybe, would be nice.

For that, blame you I cannot ?

I missed the boat for a decent seat at the Imax 3D, gonna hang on till early jan and sneak into a morning show - I'm a cranky cinema goer at the best of times.

ezietsman

It's done. Last wire soldered on Star Wars Day! Yay!

I haven't switched it on yet, went for dinner, had beer. I'll do the first startup tests and biasing tomorrow after coffee.

rikus

Wow, this flew together! Super neat work. Well done. Looking forward to your tonal impressions.

ezietsman

Yeah I have a problem, it is blowing fuses when I power it so I must bugfix now.

ezietsman

Looks like I have a bum choke, its one lead shorts to the chassis.

shaunf

eish!

ezietsman

shaunf wrote eish!

+1

I guess I'll go get a resistor and some more fuses (Yes Gearhead, I know, you were right. Learning the hard way here ? ) and then see if it'll work otherwise. Then I'll see if I can get a good quality choke from somewhere, Hammond or IG Transformatoren this time if I can. Sigh...

G-Man

Any luck troubleshooting the issue??

ezietsman

G-Man wrote Any luck troubleshooting the issue??

Hi, Yes I'm hoping the only problem is that the choke shorts to its casing, this is a massive problem because the choke is in series with the high voltage power rail and its casing is grounded, so I was dumping ±450V straight to ground which caused the fuses to blow. So now, I have to remove the choke to either repair it, have it repaired or replace it but the second bolt (with a nylock nut) is directly underneath the turretboard, so I need to lift the turretboard to get to it, I didn't feel like doing this. However, I will do so today. I will install a 5 watt 1k resistor in place of the choke for the time being, the amp will not be quite the same but it should work fine.

I also measured my heater voltages (before I had the valve rectifier installed) and those were measuring a bit low, so I will remove the 240V mains tap I was using and connect the 230V tap, this should put the voltages within spec (Wall voltage here is around 230V).

G-Man

Wow that sounds hectic! Good thing you know what you're doing coz it's sounds flippin complicated lol. Good luck!

ezietsman

G-Man wrote Wow that sounds hectic! Good thing you know what you're doing coz it's sounds flippin complicated lol. Good luck!

Not as hectic as it sounds, I'm done, just need to solder it all together again now, however, my fiancé is leaving the house now, so I won't fire it up without her being here, for health and safety reasons. I'll use the time to measure out the rest of the circuit to make sure nothing else that shouldn't be grounded is doing so etc.

shaunf

Do you not use some means of limiting voltage during your startup process?

If not, I'd strongly suggest that before you go any further, to at least build yourself a lightbulb limiter. For the less than R100 in easy to get parts, and 30 minutes of your time, it will allow you to troubleshoot at much lower voltages, as well as potentially save components what can easily be damaged by shorts and other problems.

ezietsman

shaunf wrote Do you not use some means of limiting voltage during your startup process?

If not, I'd strongly suggest that before you go any further, to at least build yourself a lightbulb limiter. For the less than R100 in easy to get parts, and 30 minutes of your time, it will allow you to troubleshoot at much lower voltages, as well as potentially save components what can easily be damaged by shorts and other problems.

No I don't :-[

Fired it up last night. No more fuses blowing but I'm getting very high V1 plate voltages which tells me something somewhere is not conducting correctly, dry joint, bad valve, missing ground wire or something. The quest continues.

ezietsman

Success!

The V1 cathode ground connection was loose and the bias supply's ground connection was dodgy. I fixed them. Normal voltages all round now, biased this bad boy up (I had to change the bias resistor to make it work), and let rip.

Loud. OMG it is loud. Very very very loud.

All my controls work. I get a bit more sponginess on the valve rectifier, The negative feedback switch makes the amp a bit stiffer/cleaner or looser/dirtier. Nice. The tonestack switch turns the amp from growling to screaming. Nice.

There's still an issue with a connection somewhere, I get pronounced hiss when the volume is dimed, removing the V2 valve kills it completely, removing V1 doesn't, so I know where to look. I'll chopstick it some more today.

Hasie

Sweet man! Excellent news!

EDIT: Samples with your attenuator or it didn't happen.

ezietsman

Hasie wrote Sweet man! Excellent news!

EDIT: Samples with your attenuator or it didn't happen.

Here you go:

Description on soundcloud. TL;DR: Amp into dummy load, speaker impulses, Tokai Love Rock, Played same part 3 times: JMP50 lead mode, JMP50 Bass mode and JTM45 Mode. Amp was otherwise left alone between takes.

shaunf

Sounds fantastic! Glad you got everything sorted. Is the choke definitely faulty or did you figure it was something else?

Do yourself a favour now and build yourself a lightbulb limiter. Very handy piece of equipment. Indispensable I would say.

So far, which combo is your favourite? Mine is most definitely the Bass tonestack with solid state rectification.

I'd really like to be able to see a larger photo of your layout, and if you like, I could suggest a couple more minor tweaks you should consider trying out.

Enjoy the amp! ?

ezietsman

shaunf wrote Sounds fantastic! Glad you got everything sorted. Is the choke definitely faulty or did you figure it was something else?

Do yourself a favour now and build yourself a lightbulb limiter. Very handy piece of equipment. Indispensable I would say.

So far, which combo is your favourite? Mine is most definitely the Bass tonestack with solid state rectification.

I'd really like to be able to see a larger photo of your layout, and if you like, I could suggest a couple more minor tweaks you should consider trying out.

Enjoy the amp! ?

Thanks! Ja the choke is done for. I don't know if I can repair it either. There's a 1k resistor in there now.

I'll take a high res picture for you later, on our way out. Basically, the specs are as follows:

split / shared cathode (switch)
slope resistor / cap: equiv of 33k+500pF / 56k + 250pF (switch)
22nF coupling caps everywhere except power section, those are 47nF
0.68µF V2a bypass
100pF bright cap
NFB is the equivalent of 27k @ 16Ω and 47k @ 8Ω (on a switch)
Filtering: 100 (50 with GZ34) - choke - 32 - 32 - 32 - 32

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