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Author Topic: help to install a power amp input to a ceriatone 18 watt amp  (Read 12109 times)
johnnierotten
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« on: January 25, 2011, 11:39:26 AM »

I've been trying to get some detailed help on this, Will anyone be able to direct me? This has to be a very simple thing for you guys.  I have a Mono signal from a seperate preamp that I want to feed into the power amp section of my 18 watt TMB/EF86 amp. Nick at ceriatone suggested using the TMB side to gain access. Will anyone tell me where to make the connections? Nick said it has to hit both sides of the phase inverter.  here is the link to the layout  . Please help me.... http://www.ceriatone.com/images/layoutPic/marshallLayout/TMBEFceriatone.jpg   
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wyatt
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« Reply #1 on: January 26, 2011, 01:44:22 AM »

Is there anyone who has a schematic?

Layouts are not for circuit design, they are for assembly.   What would take more more time then I'm willing to commit on blueprinting a layout I could find in a matter of seconds on a schematic.
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johnnierotten
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« Reply #2 on: January 26, 2011, 02:27:47 AM »

Here is the answer i recieved on th 18 watt forum,zaphod_phil
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 Posted: Wed Jan 26, 2011 9:42 am    Post subject:   

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The easiest way is just to feed your preamp into the Normal channel's input and keep the Normal channel's volume control turned down fairly low. Alternatively, if you wanted a better but slightly more complex way, it would be to feed your preamp in right after the Normal channel's volume/tone controls, and before the PI. So you would place a switching jack between the wiper of the Volume pot and the 0.01uF that comes after it.

You would certainly *not* want to "hit both sides of the phase inverter", as that would make the signal cancel itself out.
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wyatt
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« Reply #3 on: January 26, 2011, 05:33:23 AM »

Here is the answer i recieved on th 18 watt forum,zaphod_phil
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 Posted: Wed Jan 26, 2011 9:42 am    Post subject:   

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The easiest way is just to feed your preamp into the Normal channel's input and keep the Normal channel's volume control turned down fairly low. Alternatively, if you wanted a better but slightly more complex way, it would be to feed your preamp in right after the Normal channel's volume/tone controls, and before the PI. So you would place a switching jack between the wiper of the Volume pot and the 0.01uF that comes after it.

You would certainly *not* want to "hit both sides of the phase inverter", as that would make the signal cancel itself out.
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Except the EF86/TMB doesn't have the normal channel of a traditional 1974x or plain TMB, the EF86 channel replaces the normal channel and I believe it was mentioned in another thread around here recently that each channel of the EF86 feeds separately into the PI.    That's kinda why a schematic would be handy.
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johnnierotten
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« Reply #4 on: January 26, 2011, 11:27:44 AM »

This is all I could find for a schematic. Im trying guys...                      site.tubedepot.com/pdf/british18w_v6b.pdf
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wyatt
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« Reply #5 on: January 26, 2011, 04:56:32 PM »

Okay, I looked at Nik's layout (the one you posted in the opening post) and compared to Ted Weber's TMB (non-EF86!) schematic and layout.  Ted wouldn't have minded, he was an open sharing kind of guy and posted instructions on how ALL of his stuff worked, even his proprietary items).  Keep in mind the EF86 model has one extra tube, which Nik numbers V0.

https://taweber.powweb.com/store/6m18tmb_schem.jpg
https://taweber.powweb.com/store/6m18tmb_layout.jpg

So, the tube breakdown on the TMB/EF86 is as follows....going across from EF86 to rectifier...

V0...EF86 pentode gain stage EF86 channel (duh.) 
V1...12AX7
-V1A...unused triode (this could be hooked to V2B in parallel for a little fatter sound)
-V1B...first gain stage TMB channel
V2...12AX7
-V2A...PI for the EF86 channel
-V2B...PI for the TMB channel
V3...12AX7
-V3A...second gain stage for TMB channel
-V3B...driver stage for TMB channel
V4...EL84
V5...EL84
V6...EZ81

Traditionally the 12A*7 closest to the power tubes is the PI, but with a real Model 1974x, that tube is the tremolo, to reuse as much of the original layout as possible, when the TMB was designed, they left the PI where it was (V2) and used the tremolo tube as the extra TMB gain stages.  But I digress.

So, the EF86 channel uses the A half V2 for it's PI and the TMB uses the B half of V2 for it's PI.  I don't see why you would NEED to enter both halves of the PI unless you wanted an FX loop that works on both channels. 

Signal path of the EF86 channel...the signal goes from the input>>E86>>Tone control>>Volume>>PI (V2A). 

Signal path of the TMB...input>>V1B >>Volume>>V3A>>V3B>>the Tone stack, it exits the Tone stack at the Treble pot >>Master Volume>>PI (V2B)

Best place to place Poweramp In within the EF86 channel is between the Volume pot's middle lug and the turretboard.

Best Place to place the Poweramp In within the TMB channel is either between the Treble and Master Volume pots (allowing you an input Volume control from whatever preamp you are using) OR between the Master Volume pot middle lug the the turret board.

If you need to enter both sides of the PI...then you would go into V2A between the EF86 Volume and turretboard AND the TMB  channel's Master Volume and turretboard.

« Last Edit: January 26, 2011, 05:01:28 PM by wyatt » Logged
johnnierotten
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« Reply #6 on: January 26, 2011, 05:19:18 PM »

Wyatt, Thank you for taking the time to explain this situation and for the time it took to look at this. I greatly appreciate it!! It definately helps me to understand whats going on. Thanks again. John
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johnnierotten
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« Reply #7 on: January 27, 2011, 05:16:28 PM »

As a follow up  note, I did the install on the ef86 side using a 1/4"switching female socket( not sure of the exact terminology for the part)  works well!!  Thanks to wyatt for the help!   
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wyatt
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« Reply #8 on: January 27, 2011, 08:01:16 PM »

Bravo!
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johnnierotten
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« Reply #9 on: January 31, 2011, 04:12:01 PM »

After using this input, it doesnt work as well as i had originaly thought. It seems as if im overdriving the amp even with a clean signal at low volumes. Am I overdriving the phase inverter , power tubes or both? and is there a easy fix such as maybe a 12at7 tube for the PI or maybe a resister or something i could add to keep the amp from beaking up ?? thanks ,John
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johnnierotten
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« Reply #10 on: February 18, 2011, 12:10:32 PM »

Played a gig the other nite,amp was acting up.  Decided to replace the power tubes and phase inverter. Wow!   What a difference, the tubes wore out so slowly I really wasnt noticing it untill they were shot.  Now my power amp input sounds great. Just thought i would put a follow up on here in case anybody wants to know.
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