Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bmccowan on April 15, 2022, 09:23:17 am
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This morning I finished a version of Tubenit's 2x12a_7 and2xECL84. A few changes: 5Y3 rectifier; less filtering; MPS trannies; some resistor changes. It sounds really nice - thanks Tubenit. I have not tried all preamp tube options yet, but 2x12av7 sounds good, as does a 5751 in v1. I see that the OT is spec'd as 4 ohm secondary to an 8 ohm speaker. I've read pros and cons on this, so I am wondering about this. The OT I'm using is a 22K ohm primary with 4, 8, and 16 secondary leads. I'm currently using 8 to 8.
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Glad to see another SoLow Watt happening.
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Would love to hear a sound clip!
Are you getting enough bass response with this amp? I am wondering if the amp would sound better with a typical Marshall type tone stack?
I sold the one I built to a guy who opened for Derek Trucks and also later for Dave Matthews. When I checked with him quite a few yrs later, he said it was still his #1 practice amp.
Looking over your schematic, I'll add some thoughts to consider if you want to experiment .......................
- maybe remove the volume bright cap & that may allow you to remove the 500p smoothing cap across the plates on V1 and the 220p smoothing cap plate to
cathode on V2a
- try a Marshall tone stack?
- up ECL84 power cathode cap to 47uf or 22uf?
With respect, Tubenit
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Thanks Tubenit,
Funny - as I was populating the board, I thought; man, Tubenit sure likes these little caps. I removed two caps (v1 390pf plate to cathode and V2 250pf smoothing cap) the other day just to experiment and thought the amp sounded a little more "lively." The bass response is there, but it seems I have to turn up the mid some in order to give the bass more character. That limits how much I can scoop the sound. Maybe the Marshall TS would move that some. I often remove bright caps - it helps with my Tinnitus, so I will try those mods. So easy to mod with turrets.
Sound clips - I have a friend with a small studio. I'll bring it over when I get the chance - and he can play a lot better than me.
I am not surprised about the #1 practice amp comment. The volume level is comfortable driving a 12" Greenback, and the tone reminds me of 6BM8 amps which are some of my favorites.
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Tubenit - I removed the bright cap and the two smoothing caps. I like it. I also fooled with tube selection a bit and found I like 12av7 for v1, and 5751 for v2. More experiments to come on that. I'll likely try the tone stack and power tube bypass cap mods after a bit more playing time. The vol and MV interplay works really well on this amp, better than most I have used.
I am still curious about your decision to use the 4 ohm OT tap with an 8 ohm speaker? Was that driven by the particular OT you used? There is a lot that I do not understand about OTs. :laugh:
I am very fond of a Matchless Clubman 6V6 I built a while back and I wonder how that preamp would work here. I know you have built this with a pentode in V1, but it seems that a 12av7 (V1) into a 5879 (V2) would be a good topography. Lordy, if I could just stop experimenting I could actually practice getting better on guitar.
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I am still curious about your decision to use the 4 ohm OT tap with an 8 ohm speaker? Was that driven by the particular OT you used?
It's driven by not being very knowledgeable about impedance matching. :icon_biggrin: :dontknow:
I like 5879's in the V1 position. Having said that, a number of my overdrive amps use 5879's in the OD section.
I'm glad removing the "extra" smoothing caps was useful to you.
With respect, Tubenit
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I like 5879's in the V1 position. Having said that, a number of my overdrive amps use 5879's in the OD section.
Yup, I built a simpler version of one of those - you helped me. Its a nice amp. I also have a vintage GA-40 with those tubes in the first position. Love that amp.
It's driven by not being very knowledgeable about impedance matching. :icon_biggrin: :dontknow:
Ha, so we are in a similar boat on that one. I need to find a good write-up on OTs.
Thanks for always being ready to share.
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From London Power Kevin O'Connor FAQ on impedence matching
Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.
A: It is awfully expensive, and also a ridiculous suggestion. There are two issues here, though; one is the notion of “impedance matching”, and the other is simple design preference.
As stated throughout our TUT book series, speaker load impedances and reflected loads to the output tubes are all “nominal”. An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.
A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the “power bandwidth”. If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.
The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don’t behave like transistors.
The design issue for impedance matching comes into play when a designer takes the approach that “everything is critical”. In some circuits, this may be the case. Tubes don’t really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says “this is good”, “this is bad” and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.
Design approaches are dealt with in our book TUT4.Q: An “expert” suggested that I change my speakers to ones that match the highest impedance tap on my amp. How do I do this and still have the option of using a second cabinet when I play out? I think I would need three cabinets to achieve this.
A: Yes, and what a waste of your money.
Not too surprisingly, this is the same expert as in the tube-pulling/power reduction question. He really should stop talking about transformers.
Rest assured, the impedance taps on your amp are there for your convenience, to use as you will. Connecting the rated cabinet impedance to the identical rated tap selection will get you the rated power bandwidth of your amp into that load. As stated above, any “mismatch” reduces power and bandwidth, and that is all.
If you are using your 4-ohm cabinet and the 4-ohm tap, does it matter if the 16-ohm tap is being unused? Of course not. This subject is explained in detail in our book TUT3, as the “Myth of Encompassment” – a myth created purely to sell speakers and transformers. To unsuspecting players and readers of the “expert’s” column, it is no more than a scare tactic.
Transformer designers take into account the loads to be connected to the device. There is limited space in the winding window for each lamination size, and the designer wants the space to be fully utilized. The percentage of space used is the “build”. Ideally, all windings are used all the time, to keep parasitic effects to a minimum. When there is a tapped secondary, some of the secondary may not be loaded under certain conditions, so those “free” parts of the winding can potentially upset the parasitic balance. The amount of upset is usually so small as to be insignificant, even in hi-fi where such a thing might matter. In MI, there is no concern whatsoever.
In most amps, you can set the impedance selector to whatever sounds best. The one caveat is: NOT in English amps. Having replaced more Marshall OTs than anything else, I would advise that the impedance selector always be set to the rated load, or less.
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Good info - thanks!
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I'm intrigued by the changes you made in the preamp section.
For me, the switchable cathode cap on V2A ("boost") is worthwhile. Turning off the boost gives a lot more flexibility with cleaner sounds.
I meant to make the bright cap switchable but forgot to include it on the panel layout. I guess a can do it by changing the vol pot to one with push/pull switching.
For clean, turning the bass control way up sounds good. But more overdriven sounds are better with the bass control dialed way back. After reading your post I need to experiment more with the mid control.
I have not yet tried a 5751 in the preamp
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Good info Pullshocks - I plan to add that boost switch. I drilled my chassis for 2 inputs and then decided I did not need a "low" input. So a switch will go there. Funny, I do not think I tried turning the bass way up. With the bass on about 5, turning the mid up just a touch gives some nice grit to the bass, while keeping a scooped sound. Eventually I will try all combinations 12A_7s. Lets compare notes as we do so. And then, because I cannot help myself, I will rewire V2 for a pentode 5879 and try driving that from a parallel 12A_7 V1 - similar to the Matchless Clubman. A real Clubman is way too loud for me but it might suit these ECL84s.
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If you rewire V2 as a 5879, I think you're likely gonna need a cathode follower to have the tone stack work well. Options are a mosfet cathode follower OR use V1a for the 1st gain stage and V1b for the cathode follower after the 5879.
Since V1 is a paralleled 12A_7, it would make more sense to me to rewire V1 as a 5879 and leave V2 as is. However, if you like the lower gain of the 12AU7 in V1, you may not want to do that? OR ............. just move the 12AV7 into V2 with 5879 in V1
with respect, Tubenit
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Yup - I went back and looked at what I did with my Clubman preamp - parallel 6SL7 V1, octal pentode in V2 - and see that the TS is a totally different beast. Anyways, experimenting with this circuit, beyond the boost switch and changing some c&r values, is down the road a bit, as I really like the amp and should play it a while before tearing into it. :icon_biggrin: