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Post subject: Re: Bassman LTD conversion to PTP
Posted: Sun Sep 15, 2013 6:26 pm
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Post subject: Re: Bassman LTD conversion to PTP
Posted: Sun Sep 15, 2013 6:34 pm
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That's cool, for two 100-ohm resistors, in parallel, from heater line-to-ground.


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Sun Sep 15, 2013 7:07 pm
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Ok so then I have them correctly installed??


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Mon Sep 16, 2013 6:29 pm
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When reading the heater voltage, are you reading from one heater to ground or are you measuring across the two heater wires. If the former, that would explains the 3.2vac reading. Correct me if I'm wrong but shouldn't the heater voltage be measure across the two heater pins for correct reading?

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Post subject: Re: Bassman LTD conversion to PTP
Posted: Mon Sep 16, 2013 11:39 pm
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As a matter of fact you are right! Measuring across gives me 6.5vac.. Thanks for pointing this out! This means that my issue with blowing fuses is not from the heaters or in the daisy chain portion of the circuit.. I believe it has to be in the way I have the power capacitors wired up.. I have seen two different ways of wiring them which makes it confusing. Mojotone has one way that uses ground and weber wires them without using ground. Which way is correct as you can see they clearly are different.. I'm wondering if this may be the cause of the fuses popping and the tone sounding muffled?

Mojotone layout
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Weber layout
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Following the mojotone layout
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Post subject: Re: Bassman LTD conversion to PTP
Posted: Tue Sep 17, 2013 1:50 am
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Those 20mfd caps must be grounded for the PS to work properly. Usually, all four (-) leads off the caps are grounded to the chassis at one point.


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Tue Sep 17, 2013 2:00 am
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BMW2002Ti wrote:
Those 20mfd caps must be grounded for the PS to work properly. Usually, all four (-) leads off the caps are grounded to the chassis at one point.


+1

You might also check that the bias cap is correctly polarized (the "+" lead should be tied to ground).

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Post subject: Re: Bassman LTD conversion to PTP
Posted: Tue Sep 17, 2013 7:22 pm
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michaelbpuccio wrote:
I believe it has to be in the way I have the power capacitors wired up.. I have seen two different ways of wiring them which makes it confusing. Mojotone has one way that uses ground and weber wires them without using ground. Which way is correct as you can see they clearly are different.. I'm wondering if this may be the cause of the fuses popping and the tone sounding muffled?


The Weber schematic clearly shows the negative side of the power supply caps being grounded.

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Post subject: Re: Bassman LTD conversion to PTP
Posted: Wed Sep 18, 2013 1:03 pm
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Thanks for that schematic! And yes it does.. Guess I am just asking all the wrong questions....


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Wed Sep 18, 2013 1:39 pm
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michaelbpuccio,

A good starting point: measure the DC voltages at points A through E. You can have the tube in-or-out of the circuit. Only the GZ34 or WZ34 needs to be inserted. Speaker(s) connected. Volume at zero. No input (no guitar plugged in).

No tubes = slightly higher voltage readings due to lack of their load on the power supply.


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Wed Sep 18, 2013 4:24 pm
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Ok here is what I have done.. I removed the board to double check that all parts and connections on the board were where they should be. I then reinstalled the board and connected the tube side of the board making sure to follow the schematic and layout provided by mojotone. Then worked on the potentiometer side again following the schematic and layout. The part that confuses me is the input jacks because I seem to not interpret the schematic correctly..

I called mojotone and spoke with their amp tech (Steve), and he suggested that I install one tube at a time starting from the rectifier to V1 to find where the issue is with popping of the fuse. Also taking measurements.. To shorten this explanation down I found that the issue with popping fuses lies when I install V2. When I flip the power switch off the amp it blows. This does not happen when the standby switch is turned off. This also happens with V2 removed and V1 installed.. My measurements are as follows:
With Rectifier installed:
1 NA
2 473.5VDC
3 NA
4 473.5VDC
5 NA
6 370VAC
7 NA
8 370VAC
With Power tube 1 installed left 6L6:
1 Ground
2 6.6VAC (measured across to pin 7)
3 470VDC
4 470VDC
5 -56VDC
6 470VDC
7 6.6VAC (measured across to pin 2)
8 Ground
With power tube 2 installed Right 6L6:
1 Ground
2 6.6VAC
3 470VDC
4 470VDC
5 -56VDC
6 470VDC
7 6.6VAC
8 Ground
With V3 installed Left 12AX7 preamp tube:
1 293VDC
2 35.5VDC
3 56VDC
4 6.6VAC
5 6.6VAC
6 289VDC
7 38VDC
8 56VDC
9 6.6VAC
With V2 removed middle preamp tube 12AX7:
1 450VCD
2 0
3 0
4 6.6VAC
5 6.6VAC
6 456VDC
7 451VDC
8 0
9 6.6VAC
With V1 removed right preamp tube 12AX7:
1 448VDC
2 0
3 0
4 6.6VAC
5 6.6VA
6 449VDC
7 0
8 0
9 6.6VAC


Last edited by michaelbpuccio on Wed Sep 18, 2013 5:07 pm, edited 1 time in total.

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Post subject: Re: Bassman LTD conversion to PTP
Posted: Wed Sep 18, 2013 4:27 pm
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Now yes I will sacrifice a fuse to post the readings of V2 and V1 with tubes installed.. Standby........


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Wed Sep 18, 2013 4:43 pm
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Let me get this straight. You are popping a fuse when you insert V2 (12AX7B)? According to the schematic posted by Bluesky (5F6-A, Weber DIY).

There is something VERY WRONG with the wiring in that case. 12AX7 rarely pull enough current, even when the tube is completely shorted-out, to pop a main fuse.

Don't even install the output tubes, until you find the issue with the 12AX7 and or circuit around it.


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Wed Sep 18, 2013 4:49 pm
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I see the issue. You need to follow the schematic's V numbers. And tell us which tubes you are measuring.

I assume (and you know what that means), you mean V4 and V5. The 6L6GCs. And not V1 and V2 = 12AX7B.


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Post subject: Re: Bassman LTD conversion to PTP
Posted: Wed Sep 18, 2013 4:56 pm
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The schematic i have is the mojotone schematic and what i have been using.. While you posted I took the following measurements with tubes installed.. And yes there definitely has to be something wrong.... I have tried out these tubes in another amplifier and they work fine.. Could this be a problem on how I have my input jacks installed?
V2 middle pre amp tube 12AX7
1 214VDC
2 0
3 1.1VDC
4 6.6VAC
5 6.6VAC
6 365VDC
7 210VDC
8 211VDC
9 6.6VAC
V1 Right preamp tube 12AX7
1 245VDC
2 .0013VDC
3 1.9VDC
4 6.6VAC
5 6.6VAC
6 242VDC
7 1.9VDC
8 1.9VDC
9 6.6VAC


Last edited by michaelbpuccio on Wed Sep 18, 2013 5:04 pm, edited 1 time in total.

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