Tweed ODS... micro?

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nickfl
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Tweed ODS... micro?

Post by nickfl »

I recently discovered the Tweed Overdrive Special amp design over at EL34 world and I have become very intrigued by it. I have also recently been reminded how awesome the version of Robrob's bassman micro design I built a while back is, both in terms of pure tone and the ability to get it at a reasonable volume.

The tweed ODS puts together a lot of things I've been interested in lately (parallel gain stage, pentode in the preamp, and a switchable overdrive section) and making a micro version would put all that into a practical package I could live with and actually use regularly. So, I've decided to try my hand at combining them. I've taken the Tweed ODS preamp and combined it with Rob's micro power section and a LTP PI from another 12BH7 powered micro amp design I found. I figure using the LTP rather than the concertina PI in Rob's design will at worst be a waste of half of a triode section (which I would have vacant otherwise) and might just help with mimicking the sound of the big amp as it is scaled down.

So here is my initial schematic:
Tweed ODS micro schematic.png
I've gone back over it a few times and I don't think there are any errors or omissions, but this is just a draft and hasn't been built or tested. Please critique any and all of the above. As always, thanks in advance for your advice!
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10thTx
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Re: Tweed ODS... micro?

Post by 10thTx »

That's a really cool idea! I love innovative stuff. THANKS for sharing it!

I think you could use the same or similar design and values all the way to the power tubes and then use either a ECL84 push/pull for about 3-4 watts (SoLow Watt power amp).

OR ......... use same design and values to the LTPI and power tubes and then use the triodes of 6BM8's for the LTPI and pentode of the 6BM8's for about 9 watts (similar to Geezer's mini-Bassman or the HoSo56 with 6BM8's).

PLEASE keep us posted on how it turns out. I will be very interested in seeing/hearing what you come up with.

With respect, 10thtx
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nickfl
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Re: Tweed ODS... micro?

Post by nickfl »

10thTx wrote: Sat Aug 12, 2017 1:23 am That's a really cool idea! I love innovative stuff. THANKS for sharing it!

I think you could use the same or similar design and values all the way to the power tubes and then use either a ECL84 push/pull for about 3-4 watts (SoLow Watt power amp).

OR ......... use same design and values to the LTPI and power tubes and then use the triodes of 6BM8's for the LTPI and pentode of the 6BM8's for about 9 watts (similar to Geezer's mini-Bassman or the HoSo56 with 6BM8's).

PLEASE keep us posted on how it turns out. I will be very interested in seeing/hearing what you come up with.

With respect, 10thtx

Hmmm..... that is very interesting! I hadn't heard of 6BM8 tubes before, but idea of being able to use a pentode rather than a triode intrigues me and the fact that it conveniently includes a triode for the PI is very elegant.

This is exactly the sort of thing I come here looking for, thanks!
10thTx
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Re: Tweed ODS... micro?

Post by 10thTx »

The 6BM8 is still in production. It's one of my favorite tubes. It has a great bass tone for a low wattage tube. I'd aim for around 310v or less on the plates IF I was attempting this idea. I'd also use a 5751 instead of the 12AX7 in V1. And I'd eliminate some of the components between the DPDT switch and the trim of the OD (compare this schematic to original TOS).

The PPIMV will help in not overdriving the 6BM8 pentodes. It might be worth changing the 1M resistors in the LTPI to 470k to 680k range also?

I am not suggesting you change your build but simply posting this idea if someone happens to be interested? Note there is an editable ExpressSCH schematic attached.

BTW, Doug Hoffman sells layout boards for the TOS

With respect, 10thtx
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didit
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Re: Tweed ODS... micro?

Post by didit »

Excellent advice 10thtx. 6BM8 ideal as small power P-P.

Best .. Ian
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Phil_S
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Re: Tweed ODS... micro?

Post by Phil_S »

nickfl wrote: Fri Aug 11, 2017 7:48 pm ...I figure using the LTP rather than the concertina PI in Rob's design will at worst be a waste of half of a triode section (which I would have vacant otherwise) and might just help with mimicking the sound of the big amp as it is scaled down...
IMHO, you have not considered how the voltage swing and gain from the LTP will be received by a triode power tube. The one time I experimented with this, I found that the LTP sounded terrible and I concluded it was driving the triodes too hard. I also tried a paraphase inverter and had a poor result with that, too. The concertina (no gain) turned out to be perfect, with no other modifications. While I suppose my conclusion is somewhat unscientific, I think there is a good reason to use the concertina rather than the LTP. Of course, your experience may differ from mine. I suggest you try it out and see for yourself. My word of caution is to do it in a way that will allow for easy modification.

With the concertina, you can also consider using the spare triode as an extra gain stage, but that might prove to be counterproductive. I'm just guessing at this.
nickfl
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Re: Tweed ODS... micro?

Post by nickfl »

Phil_S wrote: Sat Aug 12, 2017 4:50 pm
nickfl wrote: Fri Aug 11, 2017 7:48 pm ...I figure using the LTP rather than the concertina PI in Rob's design will at worst be a waste of half of a triode section (which I would have vacant otherwise) and might just help with mimicking the sound of the big amp as it is scaled down...
IMHO, you have not considered how the voltage swing and gain from the LTP will be received by a triode power tube. The one time I experimented with this, I found that the LTP sounded terrible and I concluded it was driving the triodes too hard. I also tried a paraphase inverter and had a poor result with that, too. The concertina (no gain) turned out to be perfect, with no other modifications. While I suppose my conclusion is somewhat unscientific, I think there is a good reason to use the concertina rather than the LTP. Of course, your experience may differ from mine. I suggest you try it out and see for yourself. My word of caution is to do it in a way that will allow for easy modification.

With the concertina, you can also consider using the spare triode as an extra gain stage, but that might prove to be counterproductive. I'm just guessing at this.
Yeah, that is something that crossed my mind. I know that Rob said that for his 12au7/12bh7 micro bassman the extra gain from the LPT was not needed and he preferred the conertina. I found a LTP 12bh7 design on some guys website using a google search and he claimed it sounded good, but I don't have sound clips to consider or anything other than his subjective experience. I honestly don't have a strong enough grasp of the math yet to try to work out the voltage swing etc and decide for myself if the LTP and 12BH7s will play well together.

That being said... I'm not more and more leaning towards the 6BM8 because they will allow me to have pentode power tubes while still keeping the output power at a reasonable "play it in the house" level. And they clearly are well established as working well with a LTP PI.
nickfl
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Re: Tweed ODS... micro?

Post by nickfl »

10thTx wrote: Sat Aug 12, 2017 12:27 pm I'd also use a 5751 instead of the 12AX7 in V1. And I'd eliminate some of the components between the DPDT switch and the trim of the OD (compare this schematic to original TOS).
I'd figured I would experiment with a couple of different 12A_7 tubes in V1, V1, and the PI position, though if I build it for 6BM8s as I am now leaning toward, the PI would be part of those.

What are you reasons for suggesting eliminating the stuff between the switch and trim pot?
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romberg
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Re: Tweed ODS... micro?

Post by romberg »

Phil_S wrote: Sat Aug 12, 2017 4:50 pm
nickfl wrote: Fri Aug 11, 2017 7:48 pm ...I figure using the LTP rather than the concertina PI in Rob's design will at worst be a waste of half of a triode section (which I would have vacant otherwise) and might just help with mimicking the sound of the big amp as it is scaled down...
IMHO, you have not considered how the voltage swing and gain from the LTP will be received by a triode power tube. The one time I experimented with this, I found that the LTP sounded terrible and I concluded it was driving the triodes too hard.
One method for dealing with this I learned from a member here. Split the plate load resistors on the PI and tie the coupling caps into the center of this voltage divider. I did this on my little 2w express which was hitting the 6sn7 power tube way too hard. After I split the plate load resistors, the 6sn7 began to clip at about the same point the el34s do on my full size express.

Mike
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Phil_S
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Re: Tweed ODS... micro?

Post by Phil_S »

Mike: There is always another way. Good contribution to the thread.
Nick: If you don't mind out of production tubes, you might look for some good quality EL-95's on eBay. These can be bought cheap, name brands like Mullard and Telefunken. There is an extra advantage with EL95's because the heater requirement is very low, 200mA. It is a good sounding tube.
nickfl
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Re: Tweed ODS... micro?

Post by nickfl »

10thTx wrote: Sat Aug 12, 2017 12:27 pm The 6BM8 is still in production. It's one of my favorite tubes. It has a great bass tone for a low wattage tube. I'd aim for around 310v or less on the plates IF I was attempting this idea. I'd also use a 5751 instead of the 12AX7 in V1. And I'd eliminate some of the components between the DPDT switch and the trim of the OD (compare this schematic to original TOS).

The PPIMV will help in not overdriving the 6BM8 pentodes. It might be worth changing the 1M resistors in the LTPI to 470k to 680k range also?

I am not suggesting you change your build but simply posting this idea if someone happens to be interested? Note there is an editable ExpressSCH schematic attached.

BTW, Doug Hoffman sells layout boards for the TOS

With respect, 10thtx
What would the recommended primary impedance be for the output transformer with a pair of 6BM8s? From the datasheet it looks like it would be somewhere around 14K?
10thTx
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Re: Tweed ODS... micro?

Post by 10thTx »

Look at Geezer's original mini-bassman schematic. http://el34world.com/Forum/index.php?to ... 40#msg1540

It's a proven design and quite a few people have built one. You can do a search on EL34 World with "6BM8" and find numerous builds using that tube. However, here is a fairly recent thread about using 6BM8 tubes with some info about output transformers.

http://el34world.com/Forum/index.php?topic=21734.0

I used similar values on a 56T which was a HoSo56 with 6BM8 tubes and reverb added. I think I just used a Deluxe 5E3 OT of some type or it might have been a Hammond 1620 OT? Don't remember.

With respect, 10thtx
nickfl
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Re: Tweed ODS... micro?

Post by nickfl »

Does anyone have a thought on what the current draw will be from the 6BM8 pentodes? It looks like it would be something like 30-35ma each to me? I've googled the little wing and others and I'm seeing 100 ma as a common PT spec, but also some built with PTs as small as 60ma? I'm having trouble finding a 100 ma rated PT with low enough voltage to give me the 300ish volt plate voltage I am looking for without using a tube rectifier. I always seem to have trouble specing PTs, I usually get higher voltage than I expect for some reason.
10thTx
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Re: Tweed ODS... micro?

Post by 10thTx »

Musical Power Supplies has an PT with 190-0-190 and 120ma.

So 190 X 1.4 (solid state rectification) will be maybe 265 volts or so? That might work OK? I think you'd have to experiment a little with dropping resistors on the B+ rail to make sure V1 is getting the plate voltage that you'd want. (140 - 160v range for 12AX7. Probably less with 5751 in the same position)

They also have a 275-0-275 PT with 100ma IF you're willing to use tube rectification?

275 x 1.1 (5Y3GT) would give you about 300v.

Edcor would give you numerous options, I think? http://www.edcorusa.com/xpwrseries-1

with respect, 10thtx
nickfl
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Re: Tweed ODS... micro?

Post by nickfl »

10thTx wrote: Sat Aug 12, 2017 11:38 pm Musical Power Supplies has an PT with 190-0-190 and 120ma.

So 190 X 1.4 (solid state rectification) will be maybe 265 volts or so? That might work OK? I think you'd have to experiment a little with dropping resistors on the B+ rail to make sure V1 is getting the plate voltage that you'd want. (140 - 160v range for 12AX7. Probably less with 5751 in the same position)

They also have a 275-0-275 PT with 100ma IF you're willing to use tube rectification?

275 x 1.1 (5Y3GT) would give you about 300v.

Edcor would give you numerous options, I think? http://www.edcorusa.com/xpwrseries-1

with respect, 10thtx
Looks like edcor would be the way to go they have a 210-0-210 at 120 ma and another at 80ma, if I could get away with the lower current rating. I'll probably just spend the extra $9 on the 120ma version though since it should run cooler. I'm thinking that the 210-0-210 at 120ma unit would give me somewhere in the vicinity of 290-310v B+ with a diode rectifier, depending on loading and wall voltage? I have the impression this amp would draw something like 80ma with the 6BM8 tubes at around 300v, but thats just a rough guess. I understand that edcor rates their transformers for power under load, so I am a bit worried that I'll end up with too much voltage from the 120ma unit if I am only drawing 80-90ma.
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