Hi all!
I am new to the community and have some knowledge but not a great deal. I'm currently designing a bass amp for my friend. I've taken apart a Marshall JCM2000 TSL100 and will use the iron, valves and some comps from this to build something similar to a Gibson Thor 50 bass amp only with 4 EL34s instead of 2.
I've drawn it up and would love it if someone could maybe point out the few (kidding, many) issues there will undoubtedly be in my cut/paste efforts in design.
Couple of notes, I have 2 sets of parallel EL34s in push pull as per some JCM800 schemes I've seen and my biasing circuit in the power section is also based on the Marshall style (btw that's a 47k resistor and a 50k pot in the bottom corner there). Biasing on the preamp sections are taken from Fender amps.
Attached are my pics. Any Q's please fire away! I'll try to stay on this as closely as poss over the next few weeks.
View attachment 100W Bass Amp.pdf
View attachment gibson_thor_bass-amp_sch.pdf
I am new to the community and have some knowledge but not a great deal. I'm currently designing a bass amp for my friend. I've taken apart a Marshall JCM2000 TSL100 and will use the iron, valves and some comps from this to build something similar to a Gibson Thor 50 bass amp only with 4 EL34s instead of 2.
I've drawn it up and would love it if someone could maybe point out the few (kidding, many) issues there will undoubtedly be in my cut/paste efforts in design.
Couple of notes, I have 2 sets of parallel EL34s in push pull as per some JCM800 schemes I've seen and my biasing circuit in the power section is also based on the Marshall style (btw that's a 47k resistor and a 50k pot in the bottom corner there). Biasing on the preamp sections are taken from Fender amps.
Attached are my pics. Any Q's please fire away! I'll try to stay on this as closely as poss over the next few weeks.
View attachment 100W Bass Amp.pdf
View attachment gibson_thor_bass-amp_sch.pdf
Hi,
Your powersupply with a diodebridge, the - connected to gnd and the centralpoint of the ac winding also connected to the gnd. Two diodes of the bridge will make than an halvewave shortcitcuit of your powertransformer. This is not nice for your transformer and your diodes in the bridge. Whe using 4 outputtubes I recomment also 4 bias potmeters for biassing the output tubes. Each EL34 with a potmeter, couple capacitor, resistor from biaspotmeter and G1 seriesresistor. Make your biaspotmetercircuit so that when the wiper of the biaspotmeter makes a bad contact that the bias voltage becomes higher and not disappears.
Your powersupply with a diodebridge, the - connected to gnd and the centralpoint of the ac winding also connected to the gnd. Two diodes of the bridge will make than an halvewave shortcitcuit of your powertransformer. This is not nice for your transformer and your diodes in the bridge. Whe using 4 outputtubes I recomment also 4 bias potmeters for biassing the output tubes. Each EL34 with a potmeter, couple capacitor, resistor from biaspotmeter and G1 seriesresistor. Make your biaspotmetercircuit so that when the wiper of the biaspotmeter makes a bad contact that the bias voltage becomes higher and not disappears.
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Just my opinion, but the low voltage first stage worries me. Bass, especially as it is played these days, is VERY peaky. The attack transients are huge compared to the tone of the string. If you run out of headroom for that, your amp sounds labored and unclean. I like the effortless flow of sound from a large amp. In the days of that Thor, no one was slapping and popping, and they didn't have drummers louder than a 747.
Mississippi already pointed out the lower left side of your bridge is shorting across the transformer winding. Since you basically just copied the Thor, why did you change the rectifier from the two diode full wave to the bridge?
Why are you stacking caps in the power supply? It is one thing to stack the reservoir cap, to get a higher net voltage rating, but you have stacks on a 175v and 110v nodes. That is a waste of parts. I stock 450v caps and 500v caps, and I would just grab 450v ones for those nodes, even if 200v caps would be fine too. And ever the 355v node would be happy as a clam with a 450v cap.
And if you stack caps, you need to add parallel equalizing resistors.
And they appear to be 4.7uf? Putting those in series leaves you with effectively about 2uf at each node. That won't filter very well, nor will it decouple very well. Why not 20uf or more like most amps? Or are they 47 and the drawing is fuzzy?
Mississippi already pointed out the lower left side of your bridge is shorting across the transformer winding. Since you basically just copied the Thor, why did you change the rectifier from the two diode full wave to the bridge?
Why are you stacking caps in the power supply? It is one thing to stack the reservoir cap, to get a higher net voltage rating, but you have stacks on a 175v and 110v nodes. That is a waste of parts. I stock 450v caps and 500v caps, and I would just grab 450v ones for those nodes, even if 200v caps would be fine too. And ever the 355v node would be happy as a clam with a 450v cap.
And if you stack caps, you need to add parallel equalizing resistors.
And they appear to be 4.7uf? Putting those in series leaves you with effectively about 2uf at each node. That won't filter very well, nor will it decouple very well. Why not 20uf or more like most amps? Or are they 47 and the drawing is fuzzy?
Most manufacturers take the anode load straight from the diodes and the screen feed after the choke. Knowing EL34s, I would use separate screen grid feed resistors, in preparation for when the valves arc over inside.
6L6s don't suffer from that problem.
6L6s don't suffer from that problem.
You can take a look how Marshall generates the bias directly from HT in the 900 series amps for example.
And your bias filter caps are polarity
Cheers
Georg
And your bias filter caps are polarity
Cheers
Georg
Are TSL100 transformers known for stamina when pushed? You need more power and low end for a bass amp, of course, and I think those were known for weak transformers, no?
In a nutshell: rebuild power amp suply, including bias, and Power Amp, exactly as it was original on the Marshall, and get as creative as you wish with the Bass preamp.
Best of both Worlds.
Best of both Worlds.
Ok,I see some of the errors in my ways now! So I really need to change the rectification and add more filtering as well as some balancing resistors in parallel with any stacked caps as per marshalls. Think I just missed those off.
Those stacked caps in the “lower” voltage sections of the power supply are a result of me being a goon and not thinking about it. They should also be 47uf I for some reason wrote it as 4.7... I shall upgrade these to single caps but rated at 100uf.
Maybe this could do with being a 50w amp if the output trannies in the JcM2000 amps are a bit poor. I don’t think it’ll ever get cranked I just wanted the headroom as with bass I don’t really want any breakup.
With regards the preamp maybe it would be better to steal the AA864 bassman 50 preamp and tone stack. Aye aye aye I have some work to do! Thanks so much for your suggestions everyone it is massively appreciated. I’ll take all this away and look again at my design. Should have something to post within a couple of days. In the meantime please continue tearing my design a new butthole! 😀
Those stacked caps in the “lower” voltage sections of the power supply are a result of me being a goon and not thinking about it. They should also be 47uf I for some reason wrote it as 4.7... I shall upgrade these to single caps but rated at 100uf.
Maybe this could do with being a 50w amp if the output trannies in the JcM2000 amps are a bit poor. I don’t think it’ll ever get cranked I just wanted the headroom as with bass I don’t really want any breakup.
With regards the preamp maybe it would be better to steal the AA864 bassman 50 preamp and tone stack. Aye aye aye I have some work to do! Thanks so much for your suggestions everyone it is massively appreciated. I’ll take all this away and look again at my design. Should have something to post within a couple of days. In the meantime please continue tearing my design a new butthole! 😀
I’m really struggling to see the bias circuit in the TSL100 schematic. I think perhaps the JCM900 bias circuit could be the way to go off the HT as suggested by GeorgK.
Enzo, do you have any suggestions for the low voltage circuit? It’s just copied off the Thor and like you say, weren’t no poppin’ back then and my friend is a slappy popper if ever there was one! What can I do to sort this out? I’ve suggested changing to the Bassman 50 bass channel setup, think that’ll work?
Also thanks Jonsnell Electronic, if I’ve read this right I should give each valve a separate screen grid resistor but the anode load thing I’m not sure I understand. Would you mind clarifying what you mean? Apologies but I’m still learning. Ah hang on, you mean shift my B+ for the centre tap of the OT to before the choke. I see. Yes, something I have seen. Thanks!
Thanks again for any help friends!
Enzo, do you have any suggestions for the low voltage circuit? It’s just copied off the Thor and like you say, weren’t no poppin’ back then and my friend is a slappy popper if ever there was one! What can I do to sort this out? I’ve suggested changing to the Bassman 50 bass channel setup, think that’ll work?
Also thanks Jonsnell Electronic, if I’ve read this right I should give each valve a separate screen grid resistor but the anode load thing I’m not sure I understand. Would you mind clarifying what you mean? Apologies but I’m still learning. Ah hang on, you mean shift my B+ for the centre tap of the OT to before the choke. I see. Yes, something I have seen. Thanks!
Thanks again for any help friends!
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Your preamp is pretty generic, nothing "wrong" with Thor, I just suspect it is less suited than others. I might steal from a "bigger" amp. I am a fan of Ampegs myself, but I don't mean to steer you away from Fender.
Enzo, that’s cool it’s a good shout. I honestly don’t know the best thing to do. The Thor amp seemed simple so I just copied it. I’m not against totally reworking the circuit to be better! It’s my first build so want to keep it relatively free of extra potential problems. I did look at the V4B circuit but didn’t really look at the preamp section. I shall do that now!
Really, I’m not precious about any of the design. This is simply a case of lets build a decent bass amp with the crap I’ve got here in this TSL.
Really, I’m not precious about any of the design. This is simply a case of lets build a decent bass amp with the crap I’ve got here in this TSL.
So build the Thor and don't worry about it, it will be a learning experience, and you will have an amp you built. You will then be more aware of what the amp does well, and where it falls short, then the next thing you build will be bigger/better/cooler/ whateverer.
If you are a novice, I think a fine Ampeg like the B15N would be a great start. Not as complex as the V4B. (Which is also a fine amp, of course)
If you are a novice, I think a fine Ampeg like the B15N would be a great start. Not as complex as the V4B. (Which is also a fine amp, of course)
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