Bassman Micro EF80 puny vs large OT

Len058

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So I built a variation on @robrob Bassman Micro but I wasn't happy it sounded thin. There was enough bass for non intrusive practice but the midrange didn't have that bluesy tone. The notes do not bloom or sing. It sounded OK but the tone wasn't what you'd expect of a tube amp.
I'm just a poor guy, who can't play, so I have to practice at low volume.

Before I proceed I must thank Rob for sharing his hard work on the micro amps. Thanks to his work, I made an amazing journey through tube amp world. I now have an amp I didn't dare to dream of.

I read a lot about voltages and PT's regarding the micro builds. I see pictures of very expensive parts, being put into them but then run it through a cheap and tiny OT. I now know that's where the tone really is.

If you scroll down you'll get to the audio comparison between the Hammond 125B and the Big AskJanFirst OT.

Here it is. It still needs some bling and makeup.
IMG_20220601_215557_0.jpg


The story

After building a Madamp m15mk2 kit amp I stumbled on Rob's site and decided to build an EF80 Micro Champ. I loved the low volume tube overdrive, so I decided to convert my Madamp. It had two EL84 in push-pull, which I replaced with EF80's. Of course with the help of the Shock Brothers, so thanks guys!
I was really happy, it sounded great. But I kept looking at the Bassman Micro. The JTM45, Superlead and Bassman amps are maybe my favourite amp tones. So I had to build one. I dug through schematics, layouts and youtube video's and decided on the mods. It was going to be a JTM45 with Superlead switches. Split cathode, extra bright cap, nfb switch and the cap on V2.

The build
I made some mistakes but nothing too serious. The amp worked but there was something wrong. The Madamp kit sounded so much better. After a lot of searches, measurement and thinking, I took a good look at the tonestack. I noticed Rob's stack was not the Bassman tonestack. Working my way through Duncan's calculator I thought I found the answer. First changing the tone slope resistor to 50K and then to 33K. I had to get the resistor out of the Champ to do this. It made some improvement but not enough. The bass cap would be the next step but Duncan's calculator didn't convince me that would be the answer.
I experienced a big difference changing the tone slope in the Madamp from 56 to 33. In the Basshall it wasn't that noticeable. That was my last clue.

All the wile I had a little voice that said Tiny Output transformer. The big difference between the JTPlexieBassman and the Madamp is the output transformer, which is a 10W big iron thing. The Madamp is designed as a JTM with a bit more gain, so it shouldn't be a night and day difference. The output sections are almost identical, the Madamp has lower voltages and I even swapped all the tubes from the Madamp to the BT45LeadMan. Both Amps have PPI MV, anotherdifference is the NFB, which is absent in the Madamp, but....

The BassPlexieMan with both OT's for comparisment.
IMG_20220601_215405_7.jpg


The test
So I got my soldering iron. Took pictures of both builds and took the OT out the Madamp and put it in the BassPlexi. Before I did that, I recorded both Amps. After the swap, I recorded the FendShall again.
I recorded my guitar into my daw, with a DI and re-recorded that signal through a Re-amp box. I did my best to get the levels as close as I could get it. If you experience volume differences, their due to a difference in loudness (Luf's) not on the VU meter. During/Simultaneously recording the guitar into the DAW I recorded the 'before' of the PlexiMan.

The recording
1 Guitar into a JTM45 amp sim with 1X12 Greenback and 1 Condenser
2 Bassman Hammond 125B into Greenback, AT2020 mic 25cm from the cabinet (Yup metric! Deal with it!)
3 Madamp, as above
4 Bassman ATRA0427

5 Altering between before and after.




Question.
Would a Hammond 125C,D or E get my PlexBass sing to me like this OT, or should I just get Another 'AskJanFirst' OT? I must add that the Big OT is rated at 9K so I used the 8ohm tap for a 16Ohm speaker. It should get the load at around 18K, which is lower than Rob recommends. My voltages are also lower (285V) so that might be less critical. The Madamp has run trouble free for a while.

The madamp and a horror picture for some (ADHD, don't judge too quick)
IMG_20220116_134254_0.jpg
IMG_20220522_233655_2.jpg


If you made it this far, thanks!
Lennert
 
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mountainhick

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I am confused and lost trying to read your post:

the madamp
the Champ micro (with pair of el84s?)
a JTM45 with superlead switches
the Basshall
the JTPLexieBassman
the BT45LeadMan
the Fendshall
Bassman ATRA0427

I do want to understand.

i think the main point is that you like the beefy transformer?

And what is an "ask jan first transformer?

BTW, don't be so hard on yourself, your playing is fine.
 

Len058

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I am confused and lost trying to read your post:

the madamp
the Champ micro (with pair of el84s?)
a JTM45 with superlead switches
the Basshall
the JTPLexieBassman
the BT45LeadMan
the Fendshall
Bassman ATRA0427

I do want to understand.

i think the main point is that you like the beefy transformer?

And what is an "ask jan first transformer?

BTW, don't be so hard on yourself, your playing is fine.

Ok, sorry. Tried to entertain. All Bassman-micro references are variations on the Basman/Plexi/JTM/JMP/Superlead amp names, because I can switch between them.

The Madamp is a kit amp with push-pull el84's but I changed it to EF80's in push pull (the 'micro' power section, like the ef80 versions of the micro amps)

The Champ is the EF80 micro version, designed by Rob.

The AskJanFirst is the OT I used to replace the Hammond 125 with. It was the OT the Madamp kit came with. Jan is the manufacturer

Yes, I like the beefy transformer. I can't imagine anybody wouldn't.
 

ahiddentableau

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I don't think anybody is gonna be able to answer the question you asked. It's subjective, a matter of (your) opinion.

Here's a question for you! If you are considering buying the OT from the amp you like more ("AskJanFirst"?) I assume they are OK to swap between builds, so why can't you swap the OT you already have and try it?
 

2L man

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Welcome!

First I must say you are half wrong! You are very good guitar player!

OT which has bigger iron core produce stronger bass response and more detailed sound but slightly lower high frequency efficiency and all three are good features when good low volume sound is preferred.

So your reasonings are right!

Piemme in Italy has good selection of reasonably priced OTs for us Europeans. I think AshJanFirst are or were Italian made as well?
 

Len058

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I don't think anybody is gonna be able to answer the question you asked. It's subjective, a matter of (your) opinion.

Here's a question for you! If you are considering buying the OT from the amp you like more ("AskJanFirst"?) I assume they are OK to swap between builds, so why can't you swap the OT you already have and try it?
Well I did swap, tried and posted a recording in the OP.

Welcome!

First I must say you are half wrong! You are very good guitar player!

OT which has bigger iron core produce stronger bass response and more detailed sound but slightly lower high frequency efficiency and all three are good features when good low volume sound is preferred.

So your reasonings are right!

Piemme in Italy has good selection of reasonably priced OTs for us Europeans. I think AshJanFirst are or were Italian made as well?

Thanks!

So I just need something bigger. I looked at Piemme but I couldn't find a good option. I decided to go with the Hammond 125D. It has the specs Rob used for the design, but it's 2 steps up from what he recommends.

I hope it's good enough. The AskJanfirst (German) is very large and not quite to Rob's specs. In the Madamp kit they also used the Hammond 125E, so I presume that would be around the same size. The 125D is a bit smaller, so it would fit better at the OT spot.

I'll update after the second OT swap. If it doesn't get me there, the Hammond will go into the Madamp and the Askjanfirst will go into the Bassman. I really like this amp now.
 

robrob

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This is a 12AU7 powered Bassman Micro played in a Circus Oz performance. Sounds good to me.

 

Len058

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This is a 12AU7 powered Bassman Micro played in a Circus Oz performance. Sounds good to me.


I really can't judge an amp by listening to a recording of a cellphone. I listened to the soundcloud stuff on your site, but wasn't convinced.

I installed the 125D and I like it. I have a feeling it's a bit brighter than the ATRA-OT but that's not a bad thing. The tonestack was modded a little bit so that might contribute to the brightness.
I changed the tone slope to 56K and changed the 0,1 Bass-cap to the JTM .022. Some Bassmans had a .02uF and if you look at Duncans calculator, you see it rolls of the bass. That's a good thing in my setup. The open back Greenback loaded cab can fill the room with bass rumble. Getting rid of the real low stuff, cleans it up a bit.

Bassman old/vintage?
bassman.png


Rob's version, modern Bassman?
micro-2.png


So I did another low volume (70Db on my Phone Db app) noodle to demonstrate. Normal Channel, clean. The loop isn't looping great, my bad, sorry. I added a limiter to cut some peaks, but no real compression, EQ or other stuff. Just a light reverb from the Oceans11 pedal.
 
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robrob

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There really isn't a "Bassman old/vintage" tone stack. Fender built the 5F6A Bassman with the "modern Bassman" values. I have never seen a vintage 5F6A with the tone stack values listed on the schematic and layout diagrams. This may have been intentional when Leo stumbled upon the magic values that give us the Fender shimmer. He may have decided not to publish the magic specs and guess what? Marshall copied the schematic values when he ripped off the 5F6A.
 

Len058

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There really isn't a "Bassman old/vintage" tone stack. Fender built the 5F6A Bassman with the "modern Bassman" values. I have never seen a vintage 5F6A with the tone stack values listed on the schematic and layout diagrams. This may have been intentional when Leo stumbled upon the magic values that give us the Fender shimmer. He may have decided not to publish the magic specs and guess what? Marshall copied the schematic values when he ripped off the 5F6A.

Ok, good to know. I'm thinking about putting both caps on a switch. The .1 does make more sense for a bass amp. I am not trying to be accurate, just trying to find the tones I like without disturbing the neighbours too much.

By the way, the sound I was missing is only prominent with the Greenback speaker. My Jensen C10Q doesn't produce it nor does the speaker of a Hughes an Kettner Sixty. I can hear it on some Hendrix recordings, but it's hard to desccribe.
 

2L man

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This link you can open 125 series OT datasheets and compare them. Between A and E there is huge difference. Hammond use 1kHz as a reference power and then frequency response curves are in decibels. 125A has huge power loss on 80Hz and its highs have power gains against 1000Hz reference power. Each 3db represents half and double the power. How impedance is set has big effect as well when OT is small.

125E has normal looking frequency response and less difference between impedances. And when it is rated for higher power I think same power what A is rated comes even better?

 

mountainhick

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This link you can open 125 series OT datasheets and compare them. Between A and E there is huge difference. Hammond use 1kHz as a reference power and then frequency response curves are in decibels. 125A has huge power loss on 80Hz and its highs have power gains against 1000Hz reference power. Each 3db represents half and double the power. How impedance is set has big effect as well when OT is small.

125E has normal looking frequency response and less difference between impedances. And when it is rated for higher power I think same power what A is rated comes even better?


Looking at the frequency response graphs in the pdfs for each item is quite revealing. Thank you!

So summarizing: larger wattage and lower primary impedance each give flatter response into lower frequencies, and reduce phase shift across the useful frequency band for guitar.

i.e. generally speaking, GO BIG!
 

Len058

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This link you can open 125 series OT datasheets and compare them. Between A and E there is huge difference. Hammond use 1kHz as a reference power and then frequency response curves are in decibels. 125A has huge power loss on 80Hz and its highs have power gains against 1000Hz reference power. Each 3db represents half and double the power. How impedance is set has big effect as well when OT is small.

125E has normal looking frequency response and less difference between impedances. And when it is rated for higher power I think same power what A is rated comes even better?

Thanks for proving my findings.

So summarizing: larger wattage and lower primary impedance each give flatter response into lower frequencies, and reduce phase shift across the useful frequency band for guitar.

i.e. generally speaking, GO BIG!
Yup! The Big OT I tried was bigger and ran at 2500 ohm lower input impedance. It sounded bigger than the 125D I'm using now. The difference might be influenced by changing the tonestack but I don't think that had too much impact.

The biggest OT had that low end thump you associate with a big amp.
 




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