Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: purpletele on March 22, 2017, 12:11:25 pm
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I had become intrigued with Low Watt amps recently. I have a Maz 8 that is fantastic and I want to try a few projects to see if I can find some magic.
I had been looking at projects over in the Train Wreck Community and came across RJ Audio who had a prototype hybrid that turns out to be a derivative of Geezer's Little Wing. We worked together to come up with a parts package and the intent to document the project and include some of my little tweaks.
Thank you Geezer and all that have developed this project. Jojokeo mentioned that he had built this project along with Tubenit and probably others. I reviewed the threads that document the development.
I started the project with the intent of holding after I completed the layout and power section. I am now getting my head back into the circuit and planning an assault.
The changes that Rj's prototype included, but not limited to a SS rectifier and removing the MV. I am into tube rectifiers and wanted to include that component. I am also a big fan of the PPIMV after I implemented the MV into the Stout as detailed by Sluckey and Tubenit. I don't know if it is beneficial here or not.
I am reluctant to post the schematic that I am working on at this time today. I am essentially building the Little Wing without the MV at this time.
I have a nice TW chassis that has plenty of real estate. I was able to get some really nice transformers so I have a nice Low Watt experiment platform.
I am sure I can get this amp singing, and I am sure there are ideas and modifications that I can do from there.
Another fun project! This one is slated to receive a Red Mojotone 20" Head Cabinet.
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Looks good. Please keep us posted.
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I think you are going to have a really great sounding amp!
I am hoping to build one of these two sometime in the future using 6BM8 tubes. Not "better" then the mini-Bassman just a different approach.
I will say I like the 5879 preamp tube in these designs.
with respect, Tubenit
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I have found that the process of starting a thread has inherent responsibilities.
I have accomplished my personal goal to take some photos, edit them for Hoffman, craft a post, and then create an incentive to get the project re-started.
Thanks for the encouragement.
BTW, after I completed the mods on my Stout that you guys guided me through had made that amp quite remarkable. It was meant to be a back up amp for my colleagues but it is now being used on a recording session as the main amp, at least for now. Too funny, and quite cool.
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I have found that the process of starting a thread has inherent responsibilities.
Maybe. But how else are we gonna get our tech juice? :icon_biggrin:
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Good luck on your project PT! I will attempt to contribute to the cause if it helps at all? It does get quite the nice range of tones from a lower wattage bassman-type punchy clean on one side to the smooth grinds of a chimey vox towards the other. It was built using a Hammond donor chassis, iron, board, and sockets. It's got a few elements in common with both geezer & tubenit but had to bias cooler due to the higher voltages and I wasn't into the quick to distortion it had originally so made a few other adjustments including in the phase inverter area.
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Holy Moly,
Pressure is on now. I'm sufficiently inspired!
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I think you are going to have a really great sounding amp!
I will say I like the 5879 preamp tube in these designs.
with respect, Tubenit
I was just thinking out tubes for the Mini Wrecked Bassman. How do you think they differ from a 5751 in V1 and a 12AX7 in V2?
I have had great results with tubing, some of it by accident.
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Made some progress today, I was able to get the power section started up and tested.
Good voltages with nice glowing test tubes. I caught up to the limit of the design, so I have to cool my jets for a few days. I have other projects to do in an effort to get some momentum going.
I have to replace a second failed Pot on my 6 v 6 Plexi so I will be doing that surgery to brush up on my chops. Very strange with the Pots failing, I have since taped the openings up while I am making soldered connections. I am not sure if there was a problem with Flux dripping. Deoxit would resolve it for a short time and then sound would drop or become intermittent sounding.
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I think you are going to have a really great sounding amp!
I will say I like the 5879 preamp tube in these designs.
with respect, Tubenit
I was just thinking out tubes for the Mini Wrecked Bassman. How do you think they differ from a 5751 in V1 and a 12AX7 in V2?
I have had great results with tubing, some of it by accident.
I see now that you guys use the 5879 in V1 and the 5751 in V2. I will give that a shot.
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Moving slow on the Wrecked Bassman but making progress.
Went through an editing process with Rj on the schematic and ordered some parts.
I am going deep and trying to lay out the components from the schematic, which I am sure is simple for most around here. It certainly takes some thought and some time to understand how to transfer details of a schematic to a working circuit.
If you see any mistakes with my layout of the power section please let me know.
Obviously the drawings are in the development stage but real close.
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The are the updated drawings that we are working on, I should say, Rj is working on it.
My job is to understand the design and wire it up.
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Ugh...sorry but mistakes and design flaws. Aside from the CF stage sluckey pointed out below, you better check the power tube's bias resistor or you'll blow those expensive tubes. It's also way over-rated and a needless expense. After painstaking pointing out the importance of lead dress and grounding (RJ's using the enitre chassis as a ground plane) do NOT do this. This is basic amp building 101...please re-read the grounding info by Merlin and Aiken I sent you.
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Ugh...sorry but mistakes and design flaws. I'll let the "designer" figure them out.
This should be a very useful and educational procedure. I will be curious to discuss design issues after I get it up and running.
BV
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V2 and R7 are tangled up. Easy fix. See attached pic...
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V2 and R7 are tangled up. Easy fix. See attached pic...
Thank you, I certainly wouldn't recognize that.
BV
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you better check the power tube's bias resistor or you'll blow those expensive tubes.
I used 430R/10w on the actual HoSo56 I built yrs ago. Look at the value that Jojokeo used which is even higher.
With respect, Tubenit
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you better check the power tube's bias resistor or you'll blow those expensive tubes.
I used 430R/10w on the actual HoSo56 I built yrs ago. Look at the value that Jojokeo used which is even higher.
With respect, Tubenit
Looks like a typo on the schematic. Should be 4.7 K ,5 Watt.
Thanks guys
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Should be 4.7 K ,5 Watt.
That would be 4700R/5W. I used 430R/10w.
Please research carefully what you use for a resistor for the 6BM8 pentode power tubes.
with respect, Tubenit
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With 16Kaa load the U45-B uses a 250R cathode resistor (PT has 250v AC)
(http://i.imgur.com/vtTvwqo.jpg)
Franco
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Should be 4.7 K ,5 Watt.
That would be 4700R/5W. I used 430R/10w.
Please research carefully what you use for a resistor for the 6BM8 pentode power tubes.
with respect, Tubenit
Yikes!,
Thank you
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PT, once you get the amp running go high initially with something like 500r or 470r. Then take voltage measurements of plate to cathode then cathode to ground. Ahead of time take accurate reading of cathode resistor before powering up. Wait at least five minutes before getting plate and cathode voltage readings while amp is in play mode. Now you'll have accurate measurements and it's down to simple math to get cathode current then ultimately the tube's power dissipation. This is an easy way to know your bias in relation to your voltage and if you are using the correct value for the cathode resistor. The math part easy just don't forget to calculate for two tubes.
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PT, once you get the amp running go high initially with something like 500r or 470r. Then take voltage measurements of plate to cathode then cathode to ground. Ahead of time take accurate reading of cathode resistor before powering up. Wait at least five minutes before getting plate and cathode voltage readings while amp is in play mode. Now you'll have accurate measurements and it's down to simple math to get cathode current then ultimately the tube's power dissipation. This is an easy way to know your bias in relation to your voltage and if you are using the correct value for the cathode resistor. The math part easy just don't forget to calculate for two tubes.
Thanks Joe!
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Should be 4.7 K ,5 Watt.
That would be 4700R/5W. I used 430R/10w.
Please research carefully what you use for a resistor for the 6BM8 pentode power tubes.
with respect, Tubenit
I had the Cathode Resistor and the Screen Resistors backward in my mind. The screen resistors are 4.7K 5 Watt the Cathode Resistor is a 430 Ohm 25 Watt.
I think we are getting closer, we made a few corrections to the schematic.
BV
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Tone is certainly an issue of personal taste. So, the schematic as you have it designed may be exactly the tone you are after?
IF you find the amp to be too dark/muddy sounding & harsh (lacking smoothness) with too much compression and gain ................ I would suggest considering some of the changes I've noted on the schematic for a range of component values in key places.
I hope you will print off Geezer's mini-Bassman schematic, print off the HoSo56 with 6BM8 schematic and Joe's Cherry Bomb schematic and note the component values in those schematics compared to your latest revision. All three of those amps are proven designs.
What I would anticipate with your schematic values are this:
The paralleled V1 plate/resistor values are going to give ALOT of gain & with the 22uf cathode resistor this may sound harsh? Typically you have a ratio like 100k/1.5k , 120k/1.8k, 150k/2.2k, 180k/2.7k to 220k/3.3k. So double the 82k (since it's paralleled) and you have 164k/1.2k ratio which I would anticipate to have quite a bit of gain? IF I were using an 82k plate resistor, I'd use a 2.5k cathode resistor. OR if I was using a 1.2k cathode resistor, I'd use a 47k plate resistor IF using a 22uf cathode cap.
There is nothing wrong with having more gain there or the ratio that you have if that is what you are wanting? Note on the Little Wing mini-bassman that when he used a 82k/1.2k that he used a smaller value 2.2uf instead of a 22uf. On many high gain amps like Dumble, Mesa Boogie, Cornford, etc..... that when they are pushing higher gain that they often use smaller value coupling caps and/or smaller value cathode caps.
The .047 cap going into the LTPI will allow ALOT of bass to come thru and may sound muddy to you? Fender Super Reverb in comparison uses a .001 entrance cap into the LTPI. Maybe you were meaning to have .0047 like on the HoSo56? Vox AC30 uses a .047 there so maybe you were following that design?
The 6BM8 tubes have quite a bit of bass tone with lower .02 coupling caps so the .047 coupling caps will allow ALOT of bass and maybe muddy sounding?
The 4.7k screen resistors will add a great deal of compression to the tone. The max value of screen resistors that I have found to my liking has been 2.2k and this did add a great deal of compression with that value compared to 470R to 1k range.
With the 6BM8 pentode, the 220uf cathode cap will work but it will sound closer to a fixed biased tone then a cathode biased tone, IMO. For a smoother tone, I would suggest a much smaller value.
Again, I have no idea what tone you are looking for so the values you have chosen already may be perfect for you? I want you to be successful with your amp build and to be able to enjoy the tone for yrs to come. So, my comments are my own observations and opinions based on my personal experience and experimentation. My comments are meant to be helpful in informing you what tone your current chosen values may give you.
I think you should build the amp like you want with the tone that you want.
Genuinely with respect, Tubenit
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Tone is certainly an issue of personal taste. So, the schematic as you have it designed may be exactly the tone you are after?
IF you find the amp to be too dark/muddy sounding & harsh (lacking smoothness) with too much compression and gain ................ I would suggest considering some of the changes I've noted on the schematic for a range of component values in key places.
I hope you will print off Geezer's mini-Bassman schematic, print off the HoSo56 with 6BM8 schematic and Joe's Cherry Bomb schematic and note the component values in those schematics compared to your latest revision. All three of those amps are proven designs.
What I would anticipate with your schematic values are this:
The paralleled V1 plate/resistor values are going to give ALOT of gain & with the 22uf cathode resistor this may sound harsh? Typically you have a ratio like 100k/1.5k , 120k/1.8k, 150k/2.2k, 180k/2.7k to 220k/3.3k. So double the 82k (since it's paralleled) and you have 164k/1.2k ratio which I would anticipate to have quite a bit of gain? IF I were using an 82k plate resistor, I'd use a 2.5k cathode resistor. OR if I was using a 1.2k cathode resistor, I'd use a 47k plate resistor IF using a 22uf cathode cap.
There is nothing wrong with having more gain there or the ratio that you have if that is what you are wanting? Note on the Little Wing mini-bassman that when he used a 82k/1.2k that he used a smaller value 2.2uf instead of a 22uf. On many high gain amps like Dumble, Mesa Boogie, Cornford, etc..... that when they are pushing higher gain that they often use smaller value coupling caps and/or smaller value cathode caps.
The .047 cap going into the LTPI will allow ALOT of bass to come thru and may sound muddy to you? Fender Super Reverb in comparison uses a .001 entrance cap into the LTPI. Maybe you were meaning to have .0047 like on the HoSo56? Vox AC30 uses a .047 there so maybe you were following that design?
The 6BM8 tubes have quite a bit of bass tone with lower .02 coupling caps so the .047 coupling caps will allow ALOT of bass and maybe muddy sounding?
The 4.7k screen resistors will add a great deal of compression to the tone. The max value of screen resistors that I have found to my liking has been 2.2k and this did add a great deal of compression with that value compared to 470R to 1k range.
With the 6BM8 pentode, the 220uf cathode cap will work but it will sound closer to a fixed biased tone then a cathode biased tone, IMO. For a smoother tone, I would suggest a much smaller value.
Again, I have no idea what tone you are looking for so the values you have chosen already may be perfect for you? I want you to be successful with your amp build and to be able to enjoy the tone for yrs to come. So, my comments are my own observations and opinions based on my personal experience and experimentation. My comments are meant to be helpful in informing you what tone your current chosen values may give you.
I think you should build the amp like you want with the tone that you want.
Genuinely with respect, Tubenit
Tubenit,
Thank you providing the analysis and potential values. That is quite helpful.
I spotted what I refer to as the Tubenit Enhance Cap
Question: What voltage is recommended for the two capacitors on the legs coming off V1?
That would be C1 and C4 on The rj Wrecked Bassman Schematic dated 4.7.17.
Thank you,
BV
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What voltage is recommended for the two capacitors on the legs coming off V1? That would be C1 and C4 on The rj Wrecked Bassman Schematic dated 4.7.17.
You will likely have less then 3 volts on those cathode caps probably under 2 volts. So the listed cathode caps with 25v will be fine.
With respect, Tubenit
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Project Update:
We worked through some of the errors and omissions, it seems that the drawings match up quite well. I also have a component list that helped me double check things.
I completed the assembly and was pleasantly surprised when the amp operated with zeal upon the first flipped switch. Great strong signal with a great tone.
I like the tone of the 6BM8 tubes, a little warmer than the EL 84's, at least is seems that way to me.
The signal on the amp is so strong that the volume cannot go past 1.5 before it is too loud. The Wrecked Bassman is behaving very much like my AB763ish. We utilized a voltage divider to reduce the signal and that worked quite well. I had also installed a PPIMV on the Gies Audio design, I think it will be a good addition to this amp as well.
Any thoughts on the signal strength issue would be very welcome.
BV
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Any thoughts on the signal strength issue would be very welcome.
Use a 500k pot instead of 1M. Or tack a 1M resistor between your wiper and ground lug, does the same thing.
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Use a 500k pot instead of 1M. Or tack a 1M resistor between your wiper and ground lug, does the same thing.
I agree with that. I'd be inclined to use a 500ka pot and increase the 56k resisitor to the V2-7 grid to something more like 150k-220k.
Maybe a 5751 in V1? Maybe a 12AT7 in V2?
And then install the PPIMV.
I did note the changes you had mentioned on the 4-14-2017 "as built" PDF file. :thumbsup:
Like you, I love the tone of the 6BM8 tubes! I have never cared for the tone of the EL84's and have gotten rid of every EL84 amp I've owned or built.
With respect, Tubenit
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Thank you John and Tubenit!
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Any thoughts on the signal strength issue would be very welcome.
Use a 500k pot instead of 1M. Or tack a 1M resistor between your wiper and ground lug, does the same thing.
We I made a simple change to the volume pot without turning the amp on to see if everything was working correctly prior.
The net result was everything is powered but no sound? I changed the 1M pot back and nothing. So it appears I affected something else I am temporarily stumped.
I cannot make anything pop or even change with a chopstick, but the LTPI and associated wires are quite micro phonic.
Here is what I have done to narrow it down.
1.
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Any thoughts on the signal strength issue would be very welcome.
Use a 500k pot instead of 1M. Or tack a 1M resistor between your wiper and ground lug, does the same thing.
Well I made a simple replacement of the 1M pot and changed it to a 500K pot. I did not test the amp before I made the change.
I now have transformer hum but no sound.
Here is what I have done to trace the issue.
1. Replaced the volume pot back to 1M without change.
2. Tried another 500K Pot without change.
3. Chopstick probing resulted with no popping or anything indicating an issue.
4. The resistors and wires associated with the LTPI are micro phonic.
5. Checked voltages on the heaters harness which are good.
6. The volume knob does not affect the transformer hum.
7. The Treble knob does affect the hum
8. The middle knob does not affect the hum.
9. The bass does affect the hum slightly.
10. Replaced the pre-amp tubes without change.
Update: I checked the pots and they are good.
Slightly perplexed
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increase the 56k resisitor to the V2-7 grid to something more like 150k-220k.
the primary effect of that would be to shave off highs.
stuff a 100K audio taper pot under a 330K-470K resistor: that will tame the drive signal level. you don't want to kill the gain of the drive stage, just reduce the drive signal level to the next stage. so ~500K load is a good target.
when you put a 1M fixed R in || with a 1M pot from wiper to ground, the -3dB roll-off changes. as you increase the volume the pot's resistance falls to ~500K at full CW. so as the total resistance falls, the R-C time constant becomes variable and HP rolloff frequency changes. IOW, the LF rolloff point rises. this may not be an effect you want. the work around is to size the coupling cap at the LF rolloff point where you'd like it with R=500K.
if you use a divider, the load seen by the drive stage is constant and C is constant, so the LF rolloff point remains constant.
http://sim.okawa-denshi.jp/en/CRhikeisan.htm (http://sim.okawa-denshi.jp/en/CRhikeisan.htm)
--pete
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Thanks Dummyload,
I took three steps backward. Lost my signal and I don't know where it went.
Now I am trouble shooting for signal and I am stumped, temporarily.
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Update:
I have attached voltages and appears that I have an issue along V3/V6 which is the treble stack.
Correct me if I am wrong please.
1. Possible bad treble pot. Checking (Treble Pot is good)
2. Bad tube socket @ V2-3?
3. Bad Cap? c5, c7 or c8?
I appreciate any input
BV
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I think I'm to confused to help, couple observations;
V1 and V2 are AX7?, there is no "hiVolts" on the plates, pin 1 and 6 should be 100vdc or up. your taps sorta indicate 180 and 120.
V3 in the schemo is indicated a 6BM8, but looks to be AX7, there'es more, but til you get something over .9V on the plates, V1&2 probably won't let much sound through
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Did you check all those voltages with your meter set for AV volts?
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meter set for AV volts?
Now you're confusing me, what's AV :dontknow:
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Did you check all those voltages with your meter set for AV volts?
Did you mean DC Volts? If so, I was sure to be on DC volts.
I am looking Geezers Mini Bassman and I think I see an issue with the wiring of the TMB on this layout.
Thanks
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I was sure to be on DC volts.
If so, V1 and V2 need more than .9VDC plate to work good.
for ease, MV is typically mega-volts, mV is milli-volts
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meter set for AV volts?
Now you're confusing me, what's AV :dontknow:
Haha! AV is a finger fart.. Should had been AC.
All those voltages for v1 and v2 in that pdf could have been taken with a meter incorrectly set to AC mode.
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incorrectly set to AC mode.
I'll buy that, sir, well, maybe barter :icon_biggrin:
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When you check all plate and cathode voltages .................. does the speaker make a popping noise when you check the LTPI plate resistors but no noise when you check preamp voltages prior to the LTPI?
IF so, then the problem is probably prior to the LTPI.
Could be something as simple as a shielded wire shorting to ground or the inner wire of a shielded wire is broken in the signal chain.
When you check the plate and cathode voltages, do they look reasonable?
With respect, Tubenit
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When you check all plate and cathode voltages .................. does the speaker make a popping noise when you check the LTPI plate resistors?
IF so, then the problem is probably prior to the LTPI.
Could be something as simple as a shielded wire shorting to ground or the inner wire of a shielded wire is broken in the signal chain.
When you check the plate and cathode voltages, do they look reasonable?
With respect, Tubenit
Tubenit,
The resistors on the LTPI are very sensitive and it does generate a loud pop.
I have attached a voltage chart. I shielded wire breaking or shorting to ground is certainly possible. I have the Input wire as shielded going to V1-7 and the NFB wire going to the impedance switch is also shielded.
A broken wire would explain the sound loss after I changed the pot this morning.
UPDATE Saturday Night after all day of hunting. Bad connection on the shielded input wire. SOB
Now I can start fine tuning the amp. It still has issues and a big hum going on.
Obviously I have something wired incorrectly with the zero volts on V1-2 and V2-7.
Updated Wrecked Bassman Voltages
4.15.17
V1-1 119 V V3-1 44.2 V
V1-2 0 V V3-2 27 V
V1-3 1.1 V V3-3 19 V
V1-4 6.4 V V3-4 6.4 V
V1-5 6.4 V V3-5 6.4 V
V1-6 119 V V3-6 302 V
V1-7 0 V V3-7 291 V
V1-8 1.1 V V3-8 52.3 V
V1-9 6.4 V V3-9 179.7 V
V2-1 204 V V4-1 46.6 V
V2-2 124 V V4-2 26.76 V
V2-3 124 V V4-3 143.2 V
V2-4 6.4 V V4-4 6.4 V
V2-5 6.4 V V4-5 6.4 V
V2-6 124 V V4-6 302 V
V2-7 0 V V4-7 297 V
V2-8 1.1 V V4-8 68.7 V
V2-9 6.4 V V4-9 190 V
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Obviously I have something wired incorrectly with the zero volts on V1-2 and V2-7.
Zero is exactly what you should have. What did you do to get the other voltages back on V1 and V2?
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Obviously I have something wired incorrectly with the zero volts on V1-2 and V2-7.
Zero is exactly what you should have. What did you do to get the other voltages back on V1 and V2?
I am good with Zero Volts then.
I have a shielded cable at each end of the controls, one is the input wire and the other is the presence.
1. On the presence I moved the ground wire for the shielded wire from the Main Chassis ground to the ground lug on the pot. Didn't seem like it would make a difference but I thought it would be better etiquette. Not sure if it did anything.
2. On the shielded input wire there was some exposed copper that I could see. I hit it quickly with a clean hot tip and some love.
Unfortunately I got lazy after 8 hours of testing and cannot tell which operation made the difference.
However, I don't think the NFB ground connection would affect the voltage on the pre-amp section, so I am deducing that it was the input wire not having a good connection. That leads me to the hum which sounds like a ground issue. And that tight corner on the board may be my issue.
I used black wires on top of the board for my grounding scheme. I am jumping from the ground side of R8 to R6. I am connecting the volume ground to R6.
From R6 I jump to the negative side of C18. From C18 I jump up to negative side of C1/R3, that connects to the input jack ground.
The ground from the middle pot has a 10K resistor and that terminates at C1/R3 negative side as well.
Would you be so kind to eyeball the grounding and let me know if I have a solid path or a troublesome mish mash?
Thank you
BV
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It is unlikely this is the issue for you, but I'll toss it out there ................
One amp I built had a hum problem and it turned out that the OT wires were hooked up backwards. Instead of the typical squealing and howling you'd get ............ I just had a hum and sort of a mild oscillation.
On another amp, I had a B+ wire running too close to a OT wire and that caused a hum.
Regarding grounding issues. I always use Hoffman's grounding scheme and it has consistently given me quiet amps.
with respect, Tubenit
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It is unlikely this is the issue for you, but I'll toss it out there ................
One amp I built had a hum problem and it turned out that the OT wires were hooked up backwards. Instead of the typical squealing and howling you'd get ............ I just had a hum and sort of a mild oscillation.
On another amp, I had a B+ wire running too close to a OT wire and that caused a hum.
Regarding grounding issues. I always use Hoffman's grounding scheme and it has consistently given me quiet amps.
with respect, Tubenit
Thank you Mr. Tubenit!
I have a few more component changes and hopefully that smooth's out the tone. I really do like the 6BM8 tone, it's not as sterile as an EL 84.
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Not to seem rude as we all have builds that we chase problems with noise from one place or another.
That being said, and I have done this, I would get out a solder sucker and a hot iron and dissemble. Move that board away from the pots and input jacks. . Enough to install a preamp buss fot grounding. It is not necessary to solder to the back of the pots, but it works. I have never done it.
You may get too close to the tube sockets. Looking at what you have, I can see a number of places where hum could be an issue.
I know it sucks to consider this, but I try to think of it as simply learning. If I am building a high gain preamp my grounding is not the same as a fender. I use a star component layout which will make your leads short as possible.
Again, not dissing you, but if you want a trouble free amp for years to come, you should follow proven layout methods. Too much loose lead dress and flipping your ground from one side of the board to the other, which it appears you have done is asking for hum.
You may end up with some additiomal holes in the chassis, but if you would like I will be happy to post a photo of a build I made backwards, initially. I just thought, I will make it work. Well I chased one noise after the other until I, took it apart. Made a drawing that followed proven methods.
That was my first Marshall build. Have not had a problem with the amp since.
Just my opinion and it ain't worth near as much as the previous posters so please ignore any or, all is this.
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Ed, get a spell checker buddy! :icon_biggrin:
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Ed, get a spell checker buddy! :icon_biggrin:
Typing on a stupid pad. Sorry.
Should be better now. I just think i am too danm old to use a touch screen. You know I really do spell well. I am a writer and graphic designer. I know it is hard to believe.
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Not to seem rude as we all have builds that we chase problems with noise from one place or another.
That being said, and I have done this, I would get out a solder sucker and a hot iron and dissemble. Move that board away from the pots and input jacks. . Enough to install a preamp buss fot grounding. It is not necessary to solder to the back of the pots, but it works. I have never done it.
You may get too close to the tube sockets. Looking at what you have, I can see a number of places where hum could be an issue.
I know it sucks to consider this, but I try to think of it as simply learning. If I am building a high gain preamp my grounding is not the same as a fender. I use a star component layout which will make your leads short as possible.
Again, not dissing you, but if you want a trouble free amp for years to come, you should follow proven layout methods. Too much loose lead dress and flipping your ground from one side of the board to the other, which it appears you have done is asking for hum.
You may end up with some additiomal holes in the chassis, but if you would like I will be happy to post a photo of a build I made backwards, initially. I just thought, I will make it work. Well I chased one noise after the other until I, took it apart. Made a drawing that followed proven methods.
That was my first Marshall build. Have not had a problem with the amp since.
Just my opinion and it ain't worth near as much as the previous posters so please ignore any or, all is this.
Ed, thanks,
I decided yesterday that was going to do some reorganization to the layout which includes moving all of the tube sockets to allow me to move the board south. I wrestled with the layout and troubleshooting was difficult.
I am going to re-work the circuit the Geezer Mini-Bassman schematic. I was really looking for a smooth circuit with some headroom to get the feel of the 6BM8 tubes and I think the Mini Bassman tone stack might be the ticket for this experiment.
With a little work I can make the amp much more user friendly for my experiments with the 6BM8 tube.
Please send me the photos of your Marshall build, I am interested in the steps that you took to get stuck and the modifications to the layout that benefited the build.
Brian Vallis
otis8@sbcglobal.net
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Will do, but I but will be later.
In a nutshell, I cut the bottom off the chassis and I welded, and I but have seen them bolted. What I did was drilled the tubes backwards. Had the PT where it should be on a tube down and the tubes up. :w2:
I will show this build and you can have a good laugh, and but Sluckey is not allowed to look. Tubenit and Joe Joe are OK to look. :l2:
I will also post how I now do it, which is better. Actually, I now have 2 methods. One for low gain which is buss bar to input for preamp and transformer bolt for power. I use Merlins star for high gain since it incorporates caps right at the node. The simple reason I don't always do it this way is it is a little more time consuming and a lot of times it just is not needed.
Glad you are considering doing this. You will be much happier in the end.
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I will show this build and you can have a good laugh, and but Sluckey is not allowed to look. Tubenit and Joe Joe are OK to look. :l2:
Thanks Ed,
I am looking forward to see your evolution of style.
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> i am too danm old to use a touch screen.
BlueTooth keyboard.
Seriously. I could NOT manage passwords on the Android, and more than a few words kerekty tipped wuzz impozzible. I got a $50 Logitech and a $13 no-name. Both beat piss out of fat fingers on glass.
https://www.logitech.com/en-us/product/multi-device-keyboard-k480 (https://www.logitech.com/en-us/product/multi-device-keyboard-k480) (huh, WalMart charged me full price)
https://www.amazon.com/gp/product/B00NOE7412 (https://www.amazon.com/gp/product/B00NOE7412) (price dropped $2)
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I am not a person to be relied on for amp building advice, but I'm guessing you have grounding issues.
The quietest and best amp I have built was a homebrew that relied on two grounding points, one for the preamp and one for the power amp. Works awesome. Ugly as shit inside, too.
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> i am too danm old to use a touch screen.
BlueTooth keyboard.
I got a $50 Logitech and a $13 no-name. Both beat piss out of fat fingers on glass.
https://www.logitech.com/en-us/product/multi-device-keyboard-k480 (https://www.logitech.com/en-us/product/multi-device-keyboard-k480) (huh, WalMart charged me full price)
https://www.amazon.com/gp/product/B00NOE7412/ref=oh_aui_search_detailpage (https://www.amazon.com/gp/product/B00NOE7412/ref=oh_aui_search_detailpage) (price dropped $2)
That is a great keyboard/tool!
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I am not a person to be relied on for amp building advice, but I'm guessing you have grounding issues.
The quietest and best amp I have built was a homebrew that relied on two grounding points, one for the preamp and one for the power amp. Works awesome. Ugly as shit inside, too.
Thanks, I am sure I had a couple of issues going on. I am remodeling the layout now and I am going to build it back as Geezers Mini Bassman.
I am looking for nice smooth warm tone!
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I am not a person to be relied on for amp building advice, but I'm guessing you have grounding issues.
The quietest and best amp I have built was a homebrew that relied on two grounding points, one for the preamp and one for the power amp. Works awesome. Ugly as shit inside, too.
Thanks, I am sure I had a couple of issues going on. I am remodeling the layout now and I am going to build it back as Geezers Mini Bassman.
I am looking for nice smooth warm tone!
Sounds cool.
I actually think you had a really cool idea to begin with.
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I decided to do a little MOD to the Wrecked Bassman.
I have everything I need to revert this to The Mini Bassman. I had to give myself some room to work neatly.
Should be pretty smooth from here.
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Update: Geezers Mini-Bassman
The reconstruction project was much more enjoyable from a work space standpoint. I understand the need for really short wires on a high gain monster, but I really wanted to hear and feel the Mini Bassman circuit.
I had a few minor tweaks at start up and then the amp started to develop its tone. This amp is the quietest to date, it is really pleasant to have low noise. No issues so far.
I am going to play it for a few days and see where it goes.
This amp is really loud, I bet its overall rating is in the 12 -15 Watt Range.
My first impressions are that I like the feel of the Power Tubes, but it might be a little dark.
Mini Bassman Voltages
4.26.17
V1-1 147 V V3-1 48 V
V1-2 0 V V3-2 24 V
V1-3 1.42 V V3-3 53 mV
V1-4 0 VDC V3-4 0 VDC
V1-5 0 VDC V3-5 0 VDC
V1-6 147 V V3-6 279 V
V1-7 0 V V3-7 274 V
V1-8 1.42 V V3-8 70.2 V
V1-9 0 VDC V3-9 213.2 V
V2-1 135 V V4-1 45 V
V2-2 14 mV V4-2 24 V
V2-3 1.6 V V4-3 8.7 mV
V2-4 0 VDC V4-4 0 V
V2-5 0 VDC V4-5 0 V
V2-6 267 V V4-6 279 V
V2-7 135 V V4-7 288 V
V2-8 138 V V4-8 70.3 V
V2-9 0 VDC V4-9 203 V
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Voicing Update & Questions
The mini-bassman has some serious meat on the bone. I have a few questions that I am sure are easily answered.
1. This amp is quite powerful and in order to get full tilt boogie it needs to be cranked, which I can do when I hook it up to my Fryette Power Station (Great Tool). I seem to be a big proponent of the PPIMV.
a. If I cut in a PPIMV I need to remove the two 220K resistors and replace with a 250K or 500K Audio Dual Pot with 2.2 M resistors in line?
b. What is the Level Control that Dr. Z is using on the Cure and is that something I can try here.
2. The tone is really nice however it is a little dark. I know it may get a little darker with the PPIMV.
a. Without the PPIMV do I change the value of the 47 pF Cap splitting the 82K resistors in the power section to something higher such as 100 pF to shed the minor blanket, or do I drop a silver mica cap across the Master Pot?
b. If I cut in the PPIMV do I add two bright caps the dual pot?
3. Does anyone have any suggestions for Mods that I can try for fun? This is a perfect platform for experimenting.
Thanks for any input as always!
BV