Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mike-p on September 13, 2025, 06:22:12 am
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Hi all,
Am at the head scratching stage of a new amp build. Building from a laney lc50 mk2. Want to build the normal channel from the 6g5 as I'm slightly obsessed with the segeborn demo of the cranked brownface pro. Initially will reuse the el34 that the laney came with.
I have 2 spare pre amp tubes and am planning options. Obvious is reverb and or trem, not really interested in building with the harmonic trem and don't have enough pre amp tubes anyway. But then I was listening to some tweed demos and was thinking I could build a 5c5 1952 pro channel in as well with 12ay7 and 12ax7 PI.
I'm not finding any examples of 2 channel amps with separate pi. Obvious option would be a dpdt switch between the two channels into the PA but maybe there's a way to avoid the switch?
A single 5c5 channel would still leave a half 12ay7 unused...
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Mike, you could experiment with a pentode channel. The pentode could be cascaded into the second half of the 6G5 channel. NO need for switch but you could use AB box to switch between channels or volume pedal to blend in the pentode
Jonas
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head scratching… normal channel from the 6g5… could build a 5c5 1952 pro channel in as well with 12ay7 and 12ax7 PI… Obvious option would be a dpdt switch between the two channels into the PA but maybe there's a way to avoid the switch?
So, do you want 2 phase inverters? Or?
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Yes, 2 phase inverters going into the 6g5/ EL34 PA
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Sorry if I was unclear, as I said I'm still at the thinking stage. I am asking how two phase inverters can go into a single push pull power stage. In this hypothetical case, the normal channel of a 6g5 with ltp and the 5c5 grid leak pre and paraphrase pi ( I think that's right!) I have two input jacks available.
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To add to my unanswered questions would an ev15L be to much speaker for these circuits? Ideally I'd get a weber p15n style speaker but I'm in europe and affordable options are limited.
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Sorry if I was unclear, as I said I'm still at the thinking stage. I am asking how two phase inverters can go into a single push pull power stage. In this hypothetical case, the normal channel of a 6g5 with ltp and the 5c5 grid leak pre and paraphrase pi ( I think that's right!) I have two input jacks available.
Well, you could use up a lot of real estate including 2 noval tube sockets and two completely separate lots of parts on the main board and have a 3PDT switch to flick the 2 inputs and the (2 sets of) 2 outputs. Or see if some guy called Rob with a website has dreamed up some complicated switching concept.
But if it was my own project, I’d stick to building one of each type of Pro amp (and the result would just be a whole bunch of amps). Paraphase inverters have their own mojo. But so to amps with LTP inverters.
But complicated switch arrangements also add the potential for a whole load of extra stuff (uninvited noises etc) to troubleshoot, which might or might not appeal to you.
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Ok great thanks. I can't find any switching info on rob's site. Iwasn't going to use octal pre, just 12ay and ax7. Anyway...
I don't want to be so ambitious I never get it working... if I build the 6g5 normal channel with two 12ax7 what would be a good use of the other two pre tubes which would be less likely to cause me a headache? I could add 2 tube reverb with a bypass switch but I don't use much reverb and I like gnarly 50s tones. Maybe a second channel with trem going into the ltp as 'normal'?
Any ideas/ suggestions appreciated.
Also any thoughts on the ev speaker I mentioned appreciated too.
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Any ideas/ suggestions appreciated.
Suggestion #1: Before you begin any work at all ................... post a schematic of what you are attempting to do.
Suggestion #2: Accept that you are making enough changes from the original Fender amp you like that it won't sound the same but may still sound great.
With respect, Tubenit
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Work in progress sorry its handwritten. 6g5 normal channel with ppimv, cathode/ fixed bias and variable b plus on the PA section. I know this won't function in fixed bias mode. Will put the nfb on a switch. I want to be able to play the 6g5 normal channel in as unaltered form as possible.
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Hopefully more schematic here.
I have plenty of room in the chassis by the way
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That looks reasonable to me. The V1-b cathode resistor can be 1.5k instead of 820ohm if it's not be shared by another cathode.
I've not seen cathode/fixed bias done on an EL-34 amp before. I don't know why that wouldn't work though? I have found more often than not when I've added a switch on an amp that I find a favored position and don't actually switch it much. An example is that when I've installed pentode/triode switching .......... after some time I end up removing the switch.
You may (or may not) find that to be the case on this amp design regarding the fixed/cathode bias approach?
Just a personal preference thing for me, but over the years I've found I prefer cathode biased on my amps. When I want to make it sound more "fixed biased" in tone, I'll up the value on the cathode cap. For example, instead of maybe 25uf or 50uf, I'll use 100uf for the cathode cap on the power tubes. The larger cap value makes it sound more fixed biased to my ears. Just a thought to consider. I'm not suggesting you "should" do this.
I think the cathode biased approach you're using will work? Note how Matchless does this with EL-34's.
CLUBMAN.DSN - CLUBMAN - Page1 (https://el34world.com/charts/Schematics/files/Matchless/Matchless_clubman.pdf)
Thanks for sharing the schematic. With respect, Tubenit
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Great thanks. Took the el34 stuff and the bias switch from 18watt.com - the 36w and the vajra variant.
So now I just have two pre amp tubes to utilise. I could add the tremolo tmb channel from 18watt but I'd like something gnarly and 50s/tweedier, but sharing the ltp pi with the 6g5 channel.
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Just to be annoying, to go back to the 5c5 channel option with 2 PI's..
couldn't I treat the paraphase pi as a gain stage and either insert one side of the paraphase PI into the lpt. Or insert each OPP output from the paraphase into both sides of the ltp?
If that doesn't make sense I'll try to sketch it out.
My thinking is that the ltp has little or no gain so the 5c5 channel goes through 2pi's without much affecting the tone?
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Bias switch
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Sorry tubeswell, I'm looking hard at your switch and I can't see how it differs from the one I drew onto my schematic.
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Maybe you're pointing out I can't use the paraphase in any way with the bias switching, I see that now.
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Maybe you're pointing out I can't use the paraphase in any way with the bias switching, I see that now.
Anything is possible with more complicated switching, but that also adds potential for unwanted complications.
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So an obvious second channel would be a Tweed bassman but would that be much different in sound to the 6g5 channel?
Any suggestions for a more primitive channel as a second channel into the ltp appreciated.
Thanks all for your patience.
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I've been really enjoying the Matchless Spitfire I built recently. Very primitive preamp, just a paralleled 12AX7 with a volume control and simple tone control.
https://el34world.com/charts/Schematics/files/Matchless/Matchless_spitfire.pdf
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That would be straightforward and probably a good contrast but I should have mentioned the two amps I have built so far have been marshall 18watt derived...
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Then my next choose would be the Supro Thunderbolt preamp with the Big Muff-style tone control and the optional cascaded gain stages.
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Ooo, that might be great. Did you build it? You running it through a 15"? Really appreciate the idea thankyou
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What passaloutre said - I’ve always like the way my T-bolt builds turned out too.
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Ok I like it but that only uses the one pre tube and I'm worried I'll try to use the fourth doing something else which will cause me difficulty...I know I could just leave the hole...
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Are we talking about the 6G5 or the 6G5-A?
I've built a couple of amps using the 6G5 "center volume" tone stack. One is CF driven and the other one is anode driven. Very cool tone stack, but pretty different. I've altered the stock tone stack component values in both of my builds. The allows a mellow mid scoop and a decent mid bump. In the amp with the CF driven tone stack I even added a cap and switch to make a switchable mid boost. Let me know if you want a schematic.
/Max
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Ooo, that might be great. Did you build it? You running it through a 15"? Really appreciate the idea thankyou
I didn’t build it, I bought a vintage one that already had the mod done. And yes it sounds fabulous through the original Jensen 15”.
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Are we talking about the 6G5 or the 6G5-A?
I've built a couple of amps using the 6G5 "center volume" tone stack. One is CF driven and the other one is anode driven. Very cool tone stack, but pretty different. I've altered the stock tone stack component values in both of my builds. The allows a mellow mid scoop and a decent mid bump. In the amp with the CF tone stack driven I even added a cap and switch to make a switchable mid boost. Let me know if you want a schematic.
/Max
I'm embarrassed to say I thought the normal channels were the same... I'd like to start with it stock but now I don't know which one! This is the dream tone
i=-Ou2dP0dpmlcb9K_
I'll have to watch again to see if I can spot 5 or 6 preamp tubes, unless anyone knows which one it is.
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Ok, had a very careful look. Pretty sure 6 pre amp tubes so the -A variant I think. Also a ceramic oxford not the p15n he said in the comments so that might save me some money.
Looking at ev15l or the 15" legend speaker at the moment but seems like neither is the right option. Any suggestions?
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I like the WGS ceramic 15
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I'm guessing I should put the cathode bias cap on a switch for the thunderbolt tone?
Max, any documentation for 6g5a appreciated.
I assume I'm not trying to cram in the PI from the t bolt, please try to dissuade me from 1 tube reverb with my last pre tube!
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https://ebay.us/m/sMCfxk
Guessing these heppner alnico wouldn't be high enough wattage handling?
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I don't have any documentation for the -A revision, unfortunately.
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https://ebay.us/m/sMCfxk
Guessing these heppner alnico wouldn't be high enough wattage handling?
Doing a bit of research these seem to be cts and probably ceramic but I might give them a try.
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https://ebay.us/m/sMCfxk (https://ebay.us/m/sMCfxk)
Guessing these heppner alnico wouldn't be high enough wattage handling?
Doing a bit of research these seem to be cts and probably ceramic but I might give them a try.
The round bells are just the magnet covers. I bet they're Alnico with a 'ni' (as in 'nickel'. not 'knee' as in 'kneecap' because we don't say 'kneeckel')
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Ok I've tried to write out the cascaded t bolt channel only, I'm not sure it's correct and I'd like to know where to put a switch for the cascade?
I'm also trying to learn and understand more as I go. What is the .001uf cap to ground doing from the v1 plate?
I assume I just use the first half of the paraphase pi as a gain stage then straight in to the LTP directly before the .001 cap into the PI grid.
Lastly I'm just assuming/ hoping the 6g5a power supply section will provide roughly suitable voltages to the t bolt channel.
This is still leaving me with half a 12ax7. I think I need to come to terms with not using it...
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I think I need to come to terms with not using it...
could make it a CF to feed CH2
the .001uF shunts bad-mojo AC to ground. but I could be wrong, it's harvest season :icon_biggrin:
once you have it all dialed in, make a note to come back n tweak that cap, sometimes they stunt some good mojo also.
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I guessed it was shunting high frequency to ground to deter oscillation. Don't understand what you mean about the CF, each channel will have its own input socket.
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you have "V2a" feeding into, "driving", CH2 on the schematic. use "V2b" as a Cathode follower (CF) to "drive" that stage
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Ok I've tried to write out the cascaded t bolt channel only, I'm not sure it's correct and I'd like to know where to put a switch for the cascade?
I'm also trying to learn and understand more as I go. What is the .001uf cap to ground doing from the v1 plate?
I assume I just use the first half of the paraphase pi as a gain stage then straight in to the LTP directly before the .001 cap into the PI grid.
Lastly I'm just assuming/ hoping the 6g5a power supply section will provide roughly suitable voltages to the t bolt channel.
This is still leaving me with half a 12ax7. I think I need to come to terms with not using it...
Your schematic looks very good. I don't think you will want the first stage of the paraphase, as the LTP already has about the same amount of gain built-in.
I do think you'll want to make the input "cascade" stage switchable--most people wire the T-Bolt so that one input jack is a single triode, and the other jack goes through both jacks. It's nice to have both options, as they're very different sounds.
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Yeah I only have one jack input available for the channel for this channel as the chassis currently is. I thought ltp was unity gain and paraphase added gain?
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Paraphase and cathodyne are unity gain, which is why they almost always use the other triode for a gain stage. LTP performs gain and inversion with a single circuit, but it uses both triodes.
I find it easier to think of all three common phase inverter circuits as using two triodes to provide gain *and* inversion.
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So directly from the t bolt tone to the input cap of the lpt? Now I've got a whole preamp tube spare again!
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Cathodyne is unity gain. Paraphase has the gain of a 'normal' triode for one side, and the output taken from this is attenuated before its put into the grid of the triode on the other side - so that the output of the following triode is about the same as that of the first triode.
Edit - see attachment for another idea about what to do with the extra triode configured as a switchable single/parallel input stage. So you get different gain settings depending on which input you use, (but it's not as gainy as cascading two triodes). Or it has the added flexibility of simultaneous stereo inputs directly from your pedal board (so that you can control effect/gain options differently on your pedal board).
This tone stack (vastly different from a T-bolt) is configured to work with the V1 output to achieve a flat response when both treble and bass pots are at mid-rotation (without the 'slope' switch on), or a slight mid scoop with both pots at full rotation, or a slight mid-hump with both pots fully at cut rotation. I also found it works really good for harp if you clip out the 100pF brite cap on the volume pot
Also note that gain increases noticeably if the cathode resistor for the output tubes is fully bypassed.
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Really appreciate all this and giving me loads to think about/ figure out. Just to be clear the laney I am repurposing has 4x12ax7. My 6g5a channel uses one, one for the ltp. The tbolt pre is only using one as well, if not including any of the paraphase inverter or the cathode follower as previously suggested. That leaves the fourth...
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The Paraphase pi on the thunderbolt doesn't connect to the cathode bias of the power tubes. Couldn't I keep the whole tbolt circuit including the paraphase pi then just have a dpdt to switch either channel into the grid stoppers of the el34s? this doesn't seem too complex to me or am I being native/ over optimistic?
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The Paraphase pi on the thunderbolt doesn't connect to the cathode bias of the power tubes.
That just looks like that because the way it's drawn o n that schematic. In reality, everything with a ground symbol is connected to the signal ground
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Yes of course, I didn't follow the schematic properly. I'm trying to temper my ambition in order to succeed. Trying to build a circuit into a donor amp is already ambitious for me. Here's my plan now.
Draw up my schematic with all the trimmings.
2 channels into ltp,
bias switch and voltage regulation for the power tubes but only to be used in cathode bias mode or if changing and rebiasing for fixed bias with different tubes ie 6k6 which require lower voltage.
Cathode bias bypass cap switchable
Switchable nfb
Internal jumper switch to use both channels
Mosfet driven trem on tbolt preamp tube
1 tube reverb without transformer if possible.
THEN
Draw up a layout to fit my donor chassis WITHOUT the trem or reverb or other inadvisably tricky mods, BUT ensuring I can install them subsequently without extra difficulties due to the way i laid out the stripped back version.
Get it working without the trickier extra features first then add them if I still want them.
If anyone can point me to;
A schematic for a switch to jumper the two channels
Mosfet preamp bias trem schematic
A suitable one tube reverb, probably just for the t bolt channel, Doesn't need to provide tons of reverb but does need to sound good not just add splashy mush to the sound! I have two different spring tanks but both are for SS circuits so I assume I can't use either?
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Here is my redrawn t bolt channel. Realised I only drew the cascaded input so added a switch to hopefully bypass the cascade.
https://photos.app.goo.gl/uBPSMgVvuaVGGVmT9
Hopefully that worked couldn't get the image small enough to upload directly.
Added a mosfet trem circuit from Mr manning to the cascade stage as it had no bypass cap.
Added connections for reverb pinched from here
https://www.aggh.net/discussion/index.php?topic=50840.0
But id rather a reverb circuit without transformer. I currently have two, probably unsuitable tanks 1BB3C1B and 8DB2C1D. If I have to buy a new tank and transformer in order to use a proven circuit I will do so. I am not against using an LND150 or similar in the reverb as its just a bonus feature for me.
Any help and feedback appreciated.
Small question, what is the 270k resistor to ground directly on the first grid input doing?
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Ok, had a very careful look. Pretty sure 6 pre amp tubes so the -A variant I think. Also a ceramic oxford not the p15n he said in the comments so that might save me some money.
Looking at ev15l or the 15" legend speaker at the moment but seems like neither is the right option. Any suggestions?
Ive just sent my JBL d130f off for repairs and in the meantime ive installed a weber calif 15 ceramic in my early 60s brown vibrosonic. I like it a lot. I may not reinstall the JBL!
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Thanks, that's good to know, I have a second hand weber neomag, paper cap and cloth surround on the way for this which was a bit of luck!
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Just doodled some ideas many moons ago, not an exact fit but maybe some ideas would be useful. On the inverters, the 5C5 uses 220k plate resistors in paraphase whereas mine has one pair of 100k's, oops one should be a 82k. I drew it up with a four pole toggle swich changing the inverters and bypassing the gain stage and tone stack of the Bassman. Just utilizing the four pole switch for the inverter the switch can switch between the two as well as having 220k's for the plates (with parallel resistors to drop down to LTP values).
Other than that a cathode bias to grid leak bias on the input.
(https://i.imgur.com/LCc9Chv.png?1)
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That is really neat thanks, I'd been persuaded against complicated switching arrangements but that didn't look too complicated...argh!
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Ok, I probably won't implement the PI switch but I'm pursuing it on paper now in order to stretch my brain!
Seems like the variable grid leak /mv would not drop volume to zero as there is always 56k to ground?
The T bolt has the cathodes of the PI tube biased seperately, first 2.2k/33uf, the second 3.9k unbypassed. For the sake of simplicity (ha ha!) should I have them tied together?as in the 5c5 arrangement?
Switching cathode/ fixed bias shouldn't affect things, unless I'm missing something, your schematic doesn't seem to include power tube bias.
NFB is disconnected when switched to paraphase pi asking with the prescence control. Am I reading this correctly?
Sorry if I'm asking stupid questions or failing to understand simple aspects of the circuits. I really appreciate all the posts on this thread and I feel each time I sit down with my schematic scribblings I have a better understanding of what I'm trying to do.
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unless I'm missing something, your schematic doesn't seem to include power tube bias.
output tubes are cathode-bias in that schematic. Bias is bias (and cathode bias is a particular method of biasing). If you want fixed bias on the output tubes, then you have to figure out way of dc-isolating the output tube grids from the preceding driving stage(s) without affecting the way the paraphase inverter attenuates the signal going to the second PI stage's grid. So add more parts
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Yes sorry you've said this to me previously. The PI switch would work fine in cathode bias? I'm hoping to build the amp simple, add the bells and whistles, play it for a while and work out what I like and then hopefully remove unessassary complications. Accidentally having the paraphase and fixed bias together would not immediately cause bangs and smoke would it?
I can't see any cathode bias resistor and cap on the bassman schematic.
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What exactly is the resistor from the second half of the PI grid to ground doing. I see it is increased from 12k stock to 56k on printer2 schematic, can I increase it further? I am trying to see if I can provide the attenuated signal to the second half of the paraphase pi whilst keeping my mv/ grid leak arrangement with safety resistors.
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What exactly is the resistor from the second half of the PI grid to ground doing. I see it is increased from 12k stock to 56k on printer2 schematic, can I increase it further? I am trying to see if I can provide the attenuated signal to the second half of the paraphase pi whilst keeping my mv/ grid leak arrangement with safety resistors.
That 12k resistor is the lower leg of a 270k12k voltage divider that attenuates the signal from the plate of the 1st stage of the PI going into the grid of the 2nd stage of the PI. Google ‘voltage dividers’ to find out more.
This same voltage divider also doubles as a (270k in series with the 12k =) 282k grid leak resistor for the 6L6 being driven by the 1st triode in the PI.
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Ok great thanks, that's exactly what I thought so I'm getting somewhere mentally anyway! I'll redraw that section and post the schematic later thankyou.
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The gain structure of the T-bolt PI is all over the place. 1st triode is a fully bypassed gain stage with a 270k plate resistor and a suboptimal impedance bridge with a 282k grid leak for the following 6L6. 2nd triode also has a 270k plate resistor, but has an input signal that’s been knocked down to 4.2% of the output from the 1st stage, and is also a slightly colder-biased - and unbypassed - gain stage (hence making cathode-current-feedback that further reduces its gain), but it has a better impedance bridge going into its 6L6 than the other side does (because of the 470k grid leak resistor).
To further complicate matters, the shared 200R cathode resistor for the 6L6s is unbypassed, which acts to further reduce gain and ‘even out’ any residual imbalances.
All this is aimed at ‘swings and roundabouts’ gain alterations intended to get a ‘balanced’ push pull output stage. Only it isn’t perfectly balanced, and on top of that, the ‘balance’ becomes more imbalanced as the input signal level to the 1st PI triode increases, which is why it sounds so good.
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Really appreciate that explanation and I understood most of it I think!
He's my sketch of the PI section with variable grid leak.
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Looks like you’re going for 2 different PI circuits off the same dual triode. After staring at it for a while until my eyes went all swirly, I’d say you haven’t got enough switches to completely separate the two circuits. Having noted that, the other things I’d be concerned about with this idea are:
1) the switches are going to introduce the possibility of a bit of unwanted noise due to all the different wire runs needed, which would ruin the final experience and satisfaction rating of the build.
2) you’d need one multi-pole switch with a lot of poles to make so everything switched at once. This will be a headache to implement.
3) if you’re intent on this idea it might be a bit simpler (but not much simpler) to build 2 separate PI circuits using separate dual triodes - to cut down on the number of switch poles you need (but you’d still need at least 6 or so switch poles).
4) there’s a certain type of allure in thinking you can have one amp that does lots of tricks by adding switches for Africa but the degree of complexity needed will almost certainly beget lots of troubleshooting grief which the actual experience of can be worse than you might anticipate. So it may be simpler to give up this idea and just build one of each type of amp that you run from an ABY pedal.
Sorry to pour cold water - I don’t want to hurt any feelings. Just trying to help with my honest ‘horses mouth’ suggestions.
YMMV.
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Thanks, I need cold water in regular doses, I learnt stuff from drawing it out but I think you're right that I'm asking for trouble and misery trying to incorporate it.
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For your next schematic, may I suggest this straightforward freeware that works on most platforms. Very easy to learn to use and you can build circuits quickly and group components and save these to a folder in the program etc.
http://jschem.bplaced.net/ (http://jschem.bplaced.net/)
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The Pro is one of my all time favorites! In the tweed and black and silver panels. I never had the chance to play a brownie like in Johans video, but imagine it would be similar to my vibroverb or concert. I think you will be really happy with your choice. I just finished a resto on a 65 Pro Reverb and it has the perfect headroom/breakup for moderate volumes on stage. Set it to 10 and use the guitars volume! Love love love these amps!