Mojotone JTM45 Head Build - Voltages Question

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ESR
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Mojotone JTM45 Head Build - Voltages Question

Post by ESR » Thu Sep 28, 2017 2:55 pm

Hi all. I'm new to the forum. Have built a few amps before—a Tweed Deluxe, Tweed Vibrolux, and Blackface Vibrochamp. Now I'm moving into Marshall territory. I've ordered and received a Mojotone "British 45 Watt Style" head kit. In my experience building amps, I know enough to know how valuable it is to have "standard voltages" for reference when doing initial startup. I contacted Mojotone to ask if they had that available. They didn't have it readily available, but said they could have a tech draw it up for me and email it over. I'm waiting on that, but also looking for more references to have for the sake of comparison.

The kit I've got has the Mojotone/Heyboer transformers. And the power section is based around a pair of (JJ) E34L's. A friend tells me that this amp will be more like a JTM50 in that regard. (It does still have a tube rectifier, though.) So I'm trying to get some direction on establishing accurate reference voltages. If Mojotone sends over a thorough chart, I'll be happy to use that as my primary reference. But more info is good, IMO.

If it helps, I can cite exact transformer part numbers when I get home tonight.

Glad to be on the forum. Looking forward to any responses and help you guys can offer. Thanks in advance!

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Re: Mojotone JTM45 Head Build - Voltages Question

Post by ESR » Sat Sep 30, 2017 11:43 am

Anybody?

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Re: Mojotone JTM45 Head Build - Voltages Question

Post by danman » Sat Sep 30, 2017 5:49 pm

It's hard to give you a voltage chart without knowing which PT you will be using with your build. If it is the standard PT that Mojo supplies, maybe someone who has built the exact kit will be able to tell you what voltages they measured in their build. Otherwise, I would try a Google search. A quick search shows the old Metroamp kit instructions which has a chart included I believe...http://valvestorm.com/sites/default/fil ... T_V2.1.pdf This PT may be slightly different than the Mojo supplied one so the voltages will be slightly different. This sight...http://valvestorm.com/JTM45%20Kit may also have some info as they took over the selling of the Metroamp parts when George stopped the kits several years back. Good luck with the build and please post plenty of pics if possible.

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Re: Mojotone JTM45 Head Build - Voltages Question

Post by ESR » Sun Oct 01, 2017 12:22 am

Thanks, danman. What I know about the transformers in the kit I have from Mojotone is that they were made by Heyboer. Would it be helpful if I gave the numbers on the sides of each transformer?

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Re: Mojotone JTM45 Head Build - Voltages Question

Post by danman » Sun Oct 01, 2017 7:55 pm

I have never built that kit or used that particular PT so I really couldn't give you any concrete info. Hopefully another member that has built that particular kit will see this and chime in. Meanwhile, you could probably go with the numbers in the Metroamp guide as a general rule of thumb. The voltages don't have to match exactly but you don't want to see anything to far from normal as that may indicate a mistake.

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Re: Mojotone JTM45 Head Build - Voltages Question

Post by ESR » Mon Oct 09, 2017 3:27 pm

Thought I'd follow up on here with some specs on the transformers and choke...

Power Transformer: http://www.mojotone.com/amp-parts/Trans ... ransformer

Output Transformer: http://www.mojotone.com/amp-parts/Trans ... ransformer

Choke: http://www.mojotone.com/amp-parts/Trans ... r-6L6-Amps

If you are able to look at the specs of these transformers and give me an idea of what the standard voltages in my build ought to be, I'd greatly appreciate it. Or--even better--if you can help me understand how I can establish these values for myself based on the specs of the transformers I'm using, I'd appreciate that even more.

Thanks!

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Re: Mojotone JTM45 Head Build - Voltages Question

Post by neikeel » Tue Oct 10, 2017 12:37 pm

The OT is a 3.4k primary that you would normally use with EL34s. It is more typical to have a JTM45 with the Drake -103 OT which is an 8k primary and use KT66s, although you can use an impedance mismatch to compensate if you are going to use KT66s and chasing specific JTM45 tones (you tape off the 16ohm tap on the OT and wire the 8ohm tap to the 16 ohm on the selector and the 4ohm wire to the 8ohm on the selector) or wire all the taps up as is but remember to click down to 8ohm option when running a 16 ohm cab and KT66s, or of course use EL34s.
The PT is higher spec than an original JTM45 PT as it is for 200mA (original is 150mA) otherwise looks fine but you may get less sag at full tilt, which you may or may not prefer.
Your link is for a Fender type choke - did you mean that?
The early (Radiospares) choke is 20H and 690ohm DCR and current rating for 70mA.
Later chokes (Drake 354-114 and Dagnall C1999) are 3H with 100ohm DCR and 250mA rating.

You choose.
Neil

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Re: Mojotone JTM45 Head Build - Voltages Question

Post by ESR » Tue Oct 10, 2017 2:42 pm

neikeel wrote:
Tue Oct 10, 2017 12:37 pm
The OT is a 3.4k primary that you would normally use with EL34s. It is more typical to have a JTM45 with the Drake -103 OT which is an 8k primary and use KT66s, although you can use an impedance mismatch to compensate if you are going to use KT66s and chasing specific JTM45 tones (you tape off the 16ohm tap on the OT and wire the 8ohm tap to the 16 ohm on the selector and the 4ohm wire to the 8ohm on the selector) or wire all the taps up as is but remember to click down to 8ohm option when running a 16 ohm cab and KT66s, or of course use EL34s.
The PT is higher spec than an original JTM45 PT as it is for 200mA (original is 150mA) otherwise looks fine but you may get less sag at full tilt, which you may or may not prefer.
Your link is for a Fender type choke - did you mean that?
The early (Radiospares) choke is 20H and 690ohm DCR and current rating for 70mA.
Later chokes (Drake 354-114 and Dagnall C1999) are 3H with 100ohm DCR and 250mA rating.

You choose.
Yes, this build is from a Mojotone kit, and it's designed around EL34's. While I do enjoy the sag that comes from a tube rectifier, I personally consider it a positive thing that the 200mA spec PT will probably yield less sag.

Yes, that link for the choke is the right one. (At least that's the one that Mojotone specifically includes in this kit.) Do you think there will be problems with that?

Lastly--can you help me understand in a bit more detail how these transformer specs are useful in determining the standard voltages throughout the amp (at tube socket pins, B+, etc.)?

Thanks!

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Re: Mojotone JTM45 Head Build - Voltages Question

Post by neikeel » Tue Oct 10, 2017 4:47 pm

Unfortunately these things vary and you can get a ball park figure.

That PT will probably give you 460-470v dc with ss rectifier after the rectifier with about 460v on the plates and 455 on the screens (depending on your valves and the size of screen resistors.

After that it will depend on the schematic, how many dropping resistors of what value you use.

Probably not refer to it as JTM45, maybe say JMP50 or JTM50 style build :wink:

There is a chart here:
http://www.kbapps.com/audio/schematics/ ... pchart.php

I know it is a 100w but use the voltages in the MkII as a guide as the output voltage is similar

This is the example of wall voltage variation in a 66 JTM45 that probably needs 115vac to be at its best:http://www.marstran.com/Variac%20Voltage.htm
Neil

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Re: Mojotone JTM45 Head Build - Voltages Question

Post by danman » Tue Oct 10, 2017 7:46 pm

Like Neikeel mentioned above, so many variables such as power tube bias, tube complement and line voltage affect the final B+ that it can be hard to give concrete numbers. Have you asked Mojo if they have a chart available for this particular kit?

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Re: Mojotone JTM45 Head Build - Voltages Question

Post by ESR » Wed Oct 11, 2017 10:08 am

Yes, I asked Mojotone if they could provide standard voltages for this kit. They said they didn't have any already drawn up, but they took my email address and said they'd draw them up and send them to me. A week later I called back and was told that they were working on making standard voltages for all of their amps as a "project" to be completed by the end of the year. So who knows how long it will take for me to get them now...

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Re: Mojotone JTM45 Head Build - Voltages Question

Post by neikeel » Wed Oct 11, 2017 10:33 am

Suggest you look at the links I posted.

The forum software will not let me post the pics in full.
Neil

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Re: Mojotone JTM45 Head Build - Voltages Question

Post by ESR » Wed Oct 11, 2017 3:42 pm

Will look at them in detail. Thanks a bunch for your help!

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