Fixing a Mojotone TMB kit before I even start

[Bieworm]

Apart from all this: if it’s a JJ EZ81, take it out and throw it in the garbage for there’s a BIG chance it will fail sooner than later. I only go SS with 18 watts

If you don’t have a spare tube you should put safety diodes on it. Look online for the ceriatone 18 watt layout … I believe they are on this layout

https://ceriatone.com/british-style-18w-tmb/

[Daviedawg]

"if it’s a JJ EZ81, take it out and throw it in the garbage". Before implementing this sound advice I would play the amp for a while so you know how it sounds. If you like it then substitute the EZ81 with a different make and keep the JJ spare for emergencies.
If you swap to diodes, make sure you get fast operating ones as the standard 'cheaper by the hundred' ones may be noisy. UF4007 I think off the top of my head.

Dd

[cmatte82]

Again a huge thank you!! So I will get a new rectifier tube on order. As well as look into going SS.

I actually “built” some basic ac rectifiers in a college electronics class. So I have a very basic understanding of the concept.

But for now I think I want to keep the tube rectifier. Is there a particular one you recommend? I’ve been looking, there doesn’t seem to be a lot of options. And I know there’s not a lot of tube factories left. As I understand it JJ’s is one of the factories. Does anyone use their tubes and rebrand them? I just don’t want to buy something I think is different when it isn’t.

Yes that layout shows a couple of diodes going to the rectifier tube. I’ll get some of those also.

And I’m just thinking out loud. But I should probably get this sorted before rechecking voltages? Would a better rectifier tube possibly increase the voltages?

Regardless I won’t be playing it anymore until I get this sorted. I’ll also order some more cathode resistors. So once I get the rectifier and voltages sorted I’ll see about getting the cathode bias in spec.

Again I appreciate all this guidance.

[cmatte82]

Okay I've been doing some more "research". I've been looking at your schematic and layout for the 18w TMB design. Looks like you're using 4 uf1007 diodes and a 10w 100 ohm resistor. Has any of those values changed? The ceriatone layout shows them using 1n4007 diodes. I think those and the uf4007 are similar with the uf version being the "faster" version. Correct? Just making sure, I have a cart on mouser with some Vishay uf4007's and some Vishay 5w resistors for the cathode bias circuit and one 10w for the SS rectifier circuit.

However there is also apparently a weber copper cap SS replacement that will drop right in place of the EZ81 tube. I'm sure it's essentially those diodes and a resistor in the bottle. But it wouldn't require any mods. It's honestly about the same price as a new EZ81 tube. No need to modify anything. Then on my next build, I'll build my own SS rectifier and skip the tube altogether. Thoughts or suggestions?

And I also watched your video JMP on the SS vs Tube rectifier. I'm listening on some proper monitors in a semi treated room. And the differences were maybe even less than subtle. I think that was your point. But maybe there's a "feel" component that was lost in translation?

Lastly, what's your recommendations for 12ax7s? The ones that came in it seem fine. I only ask because I broke the tip off the top of v1 as I was showing the amp to my son. Somehow the spring in the cover wasn't centered when I first installed it, and as I took the cover either off the tube broke. Somewhere I have a some extra preamp tubes from my younger days of rolling tubes. I just haven't found it. Anyway I swapped a tube out of another amp. I noticed right off the bat the sound changed. It wasn't as full. Kind of raspy and harsh and just didn't sound great. Then after couple of minutes of jamming the volume started slowly fluctuating. I admittedly kind of freaked out at first. Then I realized I should just try another tube. So I went and borrowed another one. And all is right in the world. I jammed for a little while and it sounded like it had before. So all that to say I need a couple of preamp tubes. So let me know what you like. I don't believe in NOS tubes sounding better. I know the differences in tone can be subtle. So I'm more looking for quality and durability.

Thanks again

[Bieworm]

The copper SS rectifier is just fine. And in essence what you presume… just some diodes and maybe a sag resistor in a fancy dress.

As for the 12AX7 tubes… most would recommend the new production Mullards, but IMHO they’re just sovtek rebrands…which is fine, but you pay more for just the rebranding. I’d just buy sovtek 12AX7WA ..that’s what I use. Or EHX, also sovtek rebrands..but cheaper.
My recent builds carry Voskhod 6N2P tubes..similar to 12AX7, but different pinout..not interchangeable with 12A..7. But 20% of the price of a 12AX7

[JMPGuitars]

One other difference you should note in my SS vs. Tube video is that the voltages aren't the same coming off of them. I forgot how much, but it was probably at least 10 to 15V higher with the SS rectifier, and that can make a big difference. I didn't even them out because I like both sounds.

Yes, UF4007 is the way to go.

I would suggest making the SS/Tube switch if you want both. You can find details in my Xtra TMB docs. If you want the voltages to be equal for an equal circuit comparison, you can place Zeners and a sag resistor between the UF4007 quad and the standby switch. Sag resistor is optional.

I like new production Mullard and TungSol, though I prefer the Mullard. The Sovteks Bieworm likes are also good, but they don't all sound the same. Almost all new production tubes are made at the same factory, but they taste different. JJs aren't made there, but JJs are terrible now. Now isn't the time to stock up on tubes, prices in many cases are still double what they should be. Just get what you need, and hopefully prices will drop at some point soon. Or blow some money, your choice.

Thanks,
Josh

[chaliapin]

The copper SS rectifier is just fine. And in essence what you presume… just some diodes and maybe a sag resistor in a fancy dress.

As for the 12AX7 tubes… most would recommend the new production Mullards, but IMHO they’re just sovtek rebrands…which is fine, but you pay more for just the rebranding. I’d just buy sovtek 12AX7WA ..that’s what I use. Or EHX, also sovtek rebrands..but cheaper.
My recent builds carry Voskhod 6N2P tubes..similar to 12AX7, but different pinout..not interchangeable with 12A..7. But 20% of the price of a 12AX7

Sorry to interrupt your thread but what are the pin changes required to use the 6n2p?

[Bieworm]

The copper SS rectifier is just fine. And in essence what you presume… just some diodes and maybe a sag resistor in a fancy dress.

As for the 12AX7 tubes… most would recommend the new production Mullards, but IMHO they’re just sovtek rebrands…which is fine, but you pay more for just the rebranding. I’d just buy sovtek 12AX7WA ..that’s what I use. Or EHX, also sovtek rebrands..but cheaper.
My recent builds carry Voskhod 6N2P tubes..similar to 12AX7, but different pinout..not interchangeable with 12A..7. But 20% of the price of a 12AX7

Sorry to interrupt your thread but what are the pin changes required to use the 6n2p?

12ax7:
Pin 4+5 = heater 1
Pin 9 = heater 2

6N2P:
Pin 4 = heater 1
Pin 5= heater 2
Pin 9 = shield between 2 triodes and should be grounded to eliminate possible hum

[cmatte82]

Alright I'm back. I was waiting for the Weber rectifier to come in before changing anything else. I figure I need to get the voltages right before worrying about anything else. It finally came in. I've had the other diodes and resistors from Mouser for a while now. So all I've done so far is take some more measurements and put it in a new chart. This time with sharpie so maybe it's easier to read. And I think I understood where to put all the measurements this time. Voltages are higher, but so is the input voltages. So I'm not sure the rectifier really changed anything. And I think other than the initial B+ voltage, everything else is still lower than it should be? How do I remedy this? Change the value of the resistors across the caps? If so which way do I go? Sorry about all the questions. Here's the voltage chart.

18w Voltages 23-3-10.jpg

[JMPGuitars]

Your power tube dissipation is too high. Ignore everything else, the rest is fine for now.

You're at 108.3%, you want to be around 85%. We usually use 180R cathode resistors on modern voltages, try 180R and test again.

[cmatte82]

Will do. I didn't play it or mess with the cathode resistor yet, as I wanted to make sure the voltages were okay first. Basically I didn't want to chase my tail if changing the voltages would affect the bias. And yes I had forgot to mention I had calculated the dissipation and knew it was even further out. I do have a nice 180 ohm resistor. I'll get it put in tomorrow and report back. I also have a 150 and 160 ohm resistor also. Well I have 2 of each of all 3 values. Building up a selection for future use.

[cmatte82]

Okay so I have installed a new resistor. This got the plate dissipation down to “91.7% per tube” with the output being “11.6w and 96.7% output”. I only remeasured the necessary voltages for these calculations. I didn’t follow the voltages through the caps. Or the voltages at the preamp tubes.

Played on it briefly. I think it sounds punchier and better. But are the tubes still running too hot? Just don’t want to fry anything.

Lastly the master volume is still very touchy. It goes from off, to I can kind of hear it, to jamming with a drummer by about 2. It gets a little louder after that, but by 3-4 it’s maxed and just gets more saturated. Is this tweakable? And I don’t mind how loud it gets. In fact I kind of like that’s it’s not quite killing my ears when maxed. But I wouldn’t mind being able to dial in a lower volume jam kind of volume. Not bedroom/in the house quiet. But it really goes from a whisper to a scream without any real in between. And if it’s not tweakable I’ll get an attenuator. Just curious. And maybe the tubes are still running too hot?

Again many thanks for your guidance and help so far. I think we’re almost there. And I couldn’t have got to where I am without y’all.

p.s. I also got the tubes and parts in for the next amp. And I’ve signed up for Bruce Egnater’s amp building class in May. I think I may have a problem. Haha.

[JMPGuitars]

91% is fine, that's not that far off from our target of 85%. You should take all your voltages though. If your PI plate voltages are still low, you can reduce the 100K plate resistors. Try 82K and see if that gets you to the target. This is optional.

Roll off the guitar volume a little and see how that sounds to you when messing around. What's the volume pot resistance value?

[cmatte82]

Okay. So I swapped the 100k PI plate resistors for 82k as suggested. Then I took another round of voltages. The things I'm seeing, which may not be an issue, are

The v1a plate voltage is 194. The sheet shows 150.

V1b is a little high too at 163. The cathode voltage is only .69 and the sheet shows 1.1

V2a looks close, but v2b is just not close to the sheet at all except for plate voltage. This may be okay??

V3 now looks really close to me. It's still a little low on plate voltage. Would it be worth swapping for something around 70k? or is that close enough?

The EL84's seem to be very close too. The plates are a little higher than the sheet, but that may be fine?

I'm just not sure what the margin of error is.

And the amp still sounds quite good. It may seem a little "grainier" to me, but still not fizzy or anything. I can dial in a nice thick rock tone that I really really like.

If the voltages need some tweaking I'll gladly do that, but the main thing right now is the volume knob. I took some not so great pics. Sorry about that. The chassis was upside down and I kept unplugging so the cable wouldn't be in the way. Anyway the first two pics are the difference between that fizzy barely audible just starting to make noise volume and a decent enjoyable volume. And it takes a pretty steady hand to get there. Then the last pic is around the point of no more volume. Just more saturation. I don't mind it hitting peak volume in the 5-7 range on the dial. In fact that is probably preferable so I can dial in some power tube saturation if needed, but I wouldn't mind a little more usable range along the bottom of the dial. If possible. The pot is a 1meg audio taper. I'm really not sure what to do to change it. But I'll gladly take any advice. Thanks again

And I flipped the first two pics, but for some reason when I upload them they're still upside down?

3-14-23 Voltages.jpg

Barely On.JPEG

Decent Volume.JPEG

Max Volume.JPEG

[Bieworm]

Let your ears be your guide. When you like it, it’s ok. My tremolo TMB V1 voltages are way higher than soec’d too. That’s part of why I like it that much

[JMPGuitars]

I think typically these amps get pretty dirty by 3 or 4 on a 10 digit dial, then maybe a little more volume, then more dirt.

The question is, how do you like what hear through that sweep? If it generally sounds good, cool. If not, determine why.

Sometimes tubes themselves will cause a voltage discrepancy. If swapping V1 and V2 with each other doesn't make a big difference, you can look at the solder quality, and resistor values. Small variations (even up to 20%) aren't that big of a deal. Hitting on the 30 to 50 % variation line might change how the amp feels.

Before I considered any of that though, I would look at dropping about 15VDC from your B+ line going to "A" on the chart. If you're using a 2.2K resistor between A and B, I'd change it to a 1.5K resistor after dropping your B+, and then test your voltages again.

Thanks,
Josh

[cmatte82]

Thanks. I’ll check and see what resistor is there and look at dropping the voltage. I thought it was kind of high, but then the others seem kind of low. But swapping that resistor after lowering the initial voltage makes sense.

And as far the volume goes. Looking at the pic it’s more like on 1.5 when it hits max volume. And from when it just starts to make sound to a comfortable volume is like 2 degrees out of 360 degrees of rotation. Basically you have to either be very careful and slow, or twist it up and down multiple times to get it to a comfortable volume. It really wants to go from literally barely on to nearly full blast within maybe 5 degrees of the sweep. Fwiw I’m not trying to play it inside a house . I’m in a 30’x40’ foot shop out in the country. Too loud only bothers me. But imo it gets too loud too fast imo. I’ve never had another amp that goes from off to blasting so fast. Any guidance here would be greatly appreciated.

Thanks

[cmatte82]

Okay so here's where I'm at. I got another package from mouser today. So I did as mentioned in the dropping b+ voltages post and inserted a 15v zener after the first cap but before it went to any other part of the circuit. My resistor across to the other side of the cap was actually a 2.7k. So I ordered several sizes between 2.7k and 1.5k. But I just went with it and installed the 1.5k to start with. While I was there I also went ahead and installed the uf4007 diodes going to the rectifier as mentioned earlier in the thread. To help prevent damage if the tube rectifier goes. Right now I do still have the Weber SS rectifier replacement in there, but at some point I will try it with a better tube rectifier and see how it goes. I just neglected to order one with my last tube order. At leas the amp will be ready to use it. I think. I say that because the voltages at the rectifier pins 1 and 7 where I installed the diodes are now about half of what they were before? I guess this may make sense if they're rectifying before the actual rectifier? Which makes sense to me, I just wanted to double check that this was expected? Anyway the rest of the voltages are getting really close now. I did a quick jam on it and it still works. I actually grabbed a strat with lower output pickups. And it was a little lacking in gain, but I think that's the pickups. I should have taken one of my regular humbucker guitars. Anyway here's the voltages.

3-20-23 Voltages.jpg

If those all seem alright, then I guess my last issue is the master volume. I contacted mojotone about this too. They said to switch it out for a 1 meg audio taper pot instead of a linear. The layout and schematic call for an audio taper pot. But maybe the did send a linear one. That would make sense. I need to check that. I was too worried about the voltages and forgot to check it. Anyway they did mention I could try a 500k pot too. And it would warm up the sound some. Not sure I need that, but we'll see what I find on the pot they sent.

Thanks again.

[JMPGuitars]

Assuming you're using a 180R cathode resistor, your voltages are much better now. With a 180R, you're at 87.5% dissipation, which is great. If it's lower than 180 ohms, then they're biased too hot still.

A linear pot makes sense based on your description. It should definitely be audio taper for the volume pots. This graphic compares them pretty well:

pot-taper.jpg

I have no idea what you're doing with your rectifier, but the voltages after it are good now.

Thanks,
Josh

[cmatte82]

Thanks. Yes it’s a 180 ohm resistor. And I had checked the dissipation and thought it was good. I just forgot to mention it.

Yeah it being a linear pot would explain it. The one thing I didn’t measure before installation was the pots. And I’m pretty sure I would have noticed if it was a linear pot and it called for an audio taper. But I very well have missed it, or just assumed they knew what they were doing. But I’ll take it out tomorrow and see what it says. And maybe even measure it.

And what I did at the rectifier was install two uf4007 diodes. One each on the red wires feeding the rectifier tube from the PT. Basically like the first two diodes in your layout for the “18 watt tremolo tmb”. But they’re attached to pins 1 and 7. I even bought a stand-off like in the ceriatone layout. I mean if I got them backwards I wouldn’t have any voltage? Right?

313E0A21-62C5-44ED-9151-BAA419011907.jpeg