Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on April 14, 2010, 05:17:43 pm
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Hey guys,
I am aware that we like to occasionally color outside the lines and try some innovative amp ideas................ such as DaGeezer's amp ideas.
So, I occasionally visit other amp forums and saw this amp and sound clips on the Amp Garage under the D_mble discussion. I was VERY impressed with the sound clips and would like to build the amp. Mat (the designer of the Mini-HRM EL84) and Colossal (who drew the schematic of the amp) both gave us permission to post an editable schematic and layout version of it.
Colossal occasionally visits the Hoffman forum and was aware of DaGeezer's Little Wing & HoSo56 ...... and some of the other innovative builds here. Anyhow Colossal and Mat both seem to be very cordial individuals and I am grateful for their willingness to share this. Nice of them to do so. Check out the soundclips.
http://soundclick.com/share?songid=8927753
I plan to build a 5881 version of the amp and maybe get something around 10-12 watts which is a little more than the 56T at 7-9watts. DaGeezer is going to do his own version of this amp & I'll let him tell his own idea about that.
Editable schematic and layout here:
http://www.el34world.com/Forum/index.php?topic=9149.new#new
With respect, Tubenit
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The transformer 190-0-190 spec and the FWB rectifier don't seem to lead to 270V DC.
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Mine is going to be a 2x5881/6L6 version (Parallel SE) with one of the tube cathodes on a switch for "half power".
I will post the layout & schem as soon as I reverse the layout.......I am building it in a Marshall style head format (wanting the controls to go from left to right, chassis mounted on the bottom of the cab) & did the layout the wrong way around....DOH! :rolleyes:
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The transformer 190-0-190 spec and the FWB rectifier don't seem to lead to 270V DC.
run 1/2 secondary to FWB it sims close 270...
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Reportedly the actual amp PT is 200-0-200 and the plate was measured around 270v per the original builder using a FWB rectifier.
190v X 1.4 = 266v
I used a 250-250 PT with no center tap on an amp yrs ago with a rectifier like that and it was 350v on plates.
With respect, Tubenit
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Hey Tubenit and all, I stumbled onto this as well on TAG a couple of week ago and had to say that the sound clip was pretty convincing. I always thought that the higher preamp voltages, the balance adjust PI and a relatively clean PA was what really define alot of the *umble operating phenom so it surprised me. Hearing is believing. What do I know. :smiley: FWIW, I had rewired my Honeytone SE KT66 very similar to this a few months ago--hard wired the overdrive and ratio values to my taste--and love the sound and feel but it sure don't have that smooth bloom into harmonic trait that this one has (rather, an edgier groooowl). There is so much tone shaping going on in the HRM. Thanks for going the distance, getting the permission to disseminate here. Certainly will be interested to hear what you and Geezer get with a SE 5881. Regards
dennis
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This would be a really cool project to stuff in a Epiphone Valve Jr chassis.
Reuse the stock PT and get a beefier OT.
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This would be a really cool project to stuff in a Epiphone Valve Jr chassis.
Reuse the stock PT and get a beefier OT.
Yes, in-deedy!
I'm putting all the HRM tonestack controls on the front panel of mine (tweaker that I am, & the chassis/front panel is large @ 17" x 3")
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I added an editable SCH layout version for a D-5881 SE in the SCH library.
I am planning to install mini-pots on top of the chassis for the HRM adjustments. The pot shafts will be easily accessible.
My T-Lite head has a velcro'd front panel that stays on great but is easily removed by simply pulling it off with your hand. It allows me to change tubes etc very easily.
Just sharing some more ideas on a way to approach this.
With respect, Tubenit
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I can't really comment on the preamp design. It's a 4-stage gain machine, and looks good, with the possibility of minor tweaks after construction to make it sounds exactly the way you want.
I agree with PRR that the bridge rectifier along with 190-0-190 or 300-0-300 is confusing. If you plan on only using 1 side of a PT secondary (i.e., only from CT to one outside tap), you could do that, but you can't draw any more current than the transformer was passing when using the whole winding with a full-wave rectifier. So 190 -> bridge = ~270v, or 100-0-100 -> bridge w/ CT not used = ~280v.
You may want to ask the original designers if 270v for the preamp helped the character of the distortion.
Oddly, my biggest concern is that the effects loop is not really practical or functional. It's a good idea to have it at the end of the preamp chain to capture the sound created by the preamp, but needs to be knocked down to about 1/200th the size (guessing 20v to about 100mV) for feeding into pedals meant to gulp guitar-signals, then will likely need to be boosted back up to drive the output tube.
I think I see why you asked about 6L6 ratings/cathode resistors at 270v in the other thread.
How close can you get to 300v supply? That requires rectifying about 200-215v, and whether you use a full-wave or bridge will be determined by whether you have a 200v secondary or a 200-0-200v secondary. Assume the plate and screen will both be at 300v. For 5881, 25w/300v = 83mA, and using the triode curves (triode operation is with screen connected to plate, so both have the same voltage on them), you get 80mA with 300v on the plate at 20v of bias. Let's guess 3mA screen current at idle based on some of the data sheet conditions, so 20v/.083A = ~241 ohms, so 250 ohms is the closest standard value. Dissipation in this resistor is 1.6w, so pick a 5w resistor. Cheating from the data sheet, optimum plate load looks to be anything from 4-5k.
20v of bias means you want the preamp to be able to easily deliver at least this much. I plugged the tonestack values into the Tone Stack Calculator, and the average loss for settings midway up seems to be about 10dB, which is a reduction to about 1/3 the incoming signal. So we need at least 60v of swing from the stage before the tonestack (OD channel) and preferably more. The stage ahead of the tonestack looks to be plenty capable of doing this, and all the interstage volume pots should help you set an overall gain you like.
All-in-all, it looks good. I'd be mainly looking to be sure the PT and power supply were right, or simply grafting this preamp onto an existing amp's power section. The 269EX doesn't have the current rating to power a 5881 or 6L6; it is rated at 71mA and we'll need more than that at idle. Hammond does have a 200v CT (100-0-100) transformer rated at 115mA, but the only filament winding is 5v 2A. But they also rate their products for 115v on the primary. If your wall outlet puts 124v on this transformer, you get about 215v CT and about 5.4v on the filament winding. For the high voltage winding, ignore the CT and tape it off, and and use a bridge rectifier; for the filament winding, the voltage is about 14% low but you also won't draw the full 2A rated for the winding. The voltage will probably end up a little higher than spec, just because you'll only pull about half the rated current. Either way, it will light your tubes and work just fine. The transformer is the 263AX and I've seen it priced around $40 plus shipping. I did not see a comparable Edcor transformer that didn't have a 230v or 240v primary.
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Thanks HBP! Always great hearing from you!
Looks like I need a different type solid state rectifier with a PT having a center tap?
I plan to use an old Dano Centurian PT that is 300-0-300 that ran a p/p pair of 6V6's so I should be in good shape. I can either use a GZ34, 5V4 or go the solid state rectifier route (if I add a 5v trannie). It will have enough ma to handle a single 5881.
The effects loop is like the one I have on all 3 of my amps (going into an LTPI) including the T-Lite. I just use it only for a Boss digital delay. It works quite well, IMO in that capacity. If it doesn't work on this amp, I can very easily remove it. I should add that Mat (the amp builder/designer) has the passive effects loop in his amp.
I've got some other options for a PT if needed also. I appreciate the input. The sound clips of the amp the other guy actually built with EL84's sound really good to me
With respect, Tubenit
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I also really liked the tones and the potential in this amp! In reading the thread at Amp Garage, Mat (the designer) stated that when he tried to crank it up in a band situation, it didn't sound nearly as good. I also think it was stated that all his clips were recorded at low volume so I believe the EL84 power distortion is not contributing in a good way to the tone. I also remember reading about how the D'Lite came to be and it was stated by the designers that there was a very significant difference in tone between the Deluxe OT and a Vibrolux OT, with the larger being much better, guessing because of lack of OT saturation?
I surmise from all this that it is desired to keep the power amp as clean as possible. I think Tubenit and Dageezer are on the right track using 6L6/5881 for the output tube to get the D sound at higher volume. Is the 6L6 the "cleanest" choice?
Dageezer, how are you running parallel SE? Are you using two 5K OT's or a single 2.5K OT? I can't find a SE OT bigger than 25W.
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Dageezer, how are you running parallel SE? Are you using two 5K OT's or a single 2.5K OT?
a single 2.5K OT
I can't find a SE OT bigger than 25W.
That's plenty big for 2x6L6's SE parallel......I'm even using a 15w unit (Hammond 125ESE) in a 2x5881 SE amp with no problems at all.
The one I'm using in the HRM SE is a Weber 25w unit, just because I already have it here on the shelf.
Geezer
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have you guys kicked around any thoughts using the 6550/KT88 family? :smiley:
edit: lurking again. reading the datasheets on the 6GM5/7591 family - w/ 300V Va/Vg2 you can make 11W in SE with a single tube into a 3KΩ load. interesting... only 10Vpk required to drive to full power. 2 tubes in || SE would make ~~20W into 1K5Ω load.
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I like the idea of a 6550/KT88 to get more output/headroom. Probably be a good fit for a design like this too.
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have you guys kicked around any thoughts using the 6550/KT88 family?
Yes....I'm using a big PT intended for a Super Reverb & so will have the capability to run pretty much any tube(s) I want.
AX84 has done work on this>>>
http://www.ax84.com/static/sel/AX84_SEL_080729.pdf
Would a KT88 be "happy" @ the same loads as a 6L6??
EDIT: In doing a little investigation, it appears the 6550/KT88's want to see roughly 1/2 the load of the 6L6 family......am I correct??
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6550/KT88's want to see roughly 1/2 the load of the 6L6 family
short answer yes, but still B+ dependent...
6550 - with 250V Va/Vg2 and 1K5 Ω load makes 12.5W... run it at 270V will be fine - 1/2 the secondary load on your 2K5Ω OT for 1K2Ω... or run 2 6550 in || SE @ 270V and run OT as 2K5Ω - should make about 18W.
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A couple of questions for a single 5881 SE .............
What impedence do I want for the primary going into the OT (SE)?
This will be going into a cab with a 8 ohm Emminence Red,White & Blues.
What secondary impedence would I use? 16 ohm?
Thanks, Tubenit
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This will be going into a cab with a 8k Emminence Red,White & Blues.
Do you mean 8Ω?
What impedence do I want for the primary going into the OT (SE)?
I use 5k as a starting point....2.5k for 2x tubes parallel.
Then (with the multi-tapped secondary OT's like the big 25w Weber I have, or a 125ESE), you can try different taps & see what "sounds" best.
I know the data sheets say this-or-that for different "optimal" primary imps (depending on your plate voltage), but I just use my ears, & it always has been the 5k for a single tube that works best (for me).
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Yeah, I meant 8 ohm (or 8R) not 8k. Sorry.
So (as a starting place) try a 5k primary with 16 ohm secondary? or 8 ohm secondary?
Thanks, Tubenit
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5k primary, 8Ω secondary for 1x5881.
If you are using the 125ESE, it would be the Yellow wire for the speaker tap.
For 2x5881, it would be 2.5k primary into 8, you would use the white wire.
BTW, just sent you an email response to the schem/layout question.
G
(http://www.hammondmfg.com/125SEsch.gif)
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Excellent! Thank you!
With respect, Tubenit
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This is an eerie coincidence in that just today I stumbled onto Dumble @ the AG forum only after I was cleaning out some old links & realized I had an account set up there a long time ago. I check an e/m response here then notice the Dumble thread that's just started w/ Geezer planning away, tubenit conjuring up ideas w/ schematics & layouts...you guys are amazing. I mean that as a total complement.
I d/l'd many of the pictures & schems there and found a number of interesting things from confirming tranny positions to his way of layouts. That letter w/ his price list & security agreement demonstrates his ego & eccentricism.
I just see where he's cascading two extra gain stages instead of using a CF like we've been doing lately similar to the HoSo and 56T to name a few. With a few extra tone shaping trimmers and bright caps w/ switches, I don't see where there's a really big deal or secret or magic to the tone associated w/ those amps? Am I wrong?
I actually kind of like the idea that Merlin has in alternating the bias stages btwn hot & cold to produce cut-off & grid current limiting generating sustain & harmonic complexity along the way. Are there some amp examples w/ soundbites for this kind of design & tone to hear as a comparrison?
Is this (harmonic content) why geezer & others are going w/ SE amp designs for this dumble idea? If so, I thought that a clean PA was what contributed to & was sought after for this type of amp emphasizing the preamp gain? What about the issue associated noise inherent w/ SE amps & high gain stages?
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What about the issue associated noise inherent w/ SE amps & high gain stages?
I am planning on having a quiet amp. What & where have you heard about this as an issue?
With respect, Tubenit
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If you are talking about the "single-ended hummmmm", then that can easily be eliminated with proper filtering of the power supply. I use a "pi" filter on my SE amps & don't have a problem with hum or noise.
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Here is DaGeezer's version of the mini-D HRM. The editable schematic is in the SCH Library.
With respect, Tubenit
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> Would a KT88 be "happy" @ the same loads as a 6L6??
At the same voltage and current, yes.
> appears the 6550/KT88's want to see roughly 1/2 the load of the 6L6 family......am I correct??
6550 is a much bigger and more expensive tube than 6L6. Don't use it unless you need it; if you need it, USE it. That means working it harder than the ~~24W diss which is safe for 6L6GC. Raise the voltage or raise the current. A hard-worked 6L6 already likes 400V, which is close-enough to the usual 450V cap rating. To get a 6550 to "more", you would typically leave the voltage similar and increase the current. Then reduce the load impedance to take advantage of the higher current.
As a rough-cut: find a voltage and current which does not need any exotic parts (my 560 Volt 6550 amp's power supply was an awkward stack of 350V caps) and which hits your intended dissipation (NOS and good Russian 6550 will stand an honest 40 Watts). Then divide voltage by current and call it your load.
Most of the classic audio tubes WILL make large output with not a lot over 250V. Unlike fix-bias push-pull amps, over-volting does not significantly increase power. (My 560V 6550 amp would not hit the claim in the 6550 sheet for 400V supply.)
Using an example above: 300v and 83mA gives 25 Watts, OK for 6L6GC KT66 EL34 etc. 300V/83mA= 3,614 ohms, try 4K load.
Using Hammond 125ESE, 2.5K/5K/10K and a 6550:
324V 130mA is 42 Watts and 2.5K load
458V 92mA is 42 Watts and 5K load
648V 65mA is 42 Watts and 10K load
That "42W" is dissipation, idle heat. The actual audio output will be 0.25 to 0.4 times the dissipation, 10W to 17W. I've had 13W clean 17W not-too-bent from a 6550 at 39 Watts.
Since 125ESE is rated 80mA max, the first plan is over-spec. The second can be fudged to 80mA 400V 32 Watts (and IMHO 450V 90mA will be fine also, assuming 500V caps are handy). I've tried the 600V 60mA 10K zone and do not think you want to go there.
> I can't find a SE OT bigger than 25W.
There's a 75W in Hammond. Costs $300.
The real deal is: you do not want to LIFT an SE amp at 25W or above. Figure iron at 5 watts per pound. The OT should weigh 5 pounds. The PT needs 7.5 pounds for plate power, and a few pounds more for heaters. Such heavy lumps will need a sturdy chassis. SE needs good filtering, and at this power level that adds significant weight. You might note the OT and PT weigh more than the iron for a 40W Bassman.
Maybe you can't put a price/weight limit on "tone". But an 80 pound beast will limit how many places you will actually use that tone.
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6550 is a much bigger and more expensive tube than 6L6. Don't use it unless you need it
I doubt I'll need it, as the 2x 6L6's should be a-plenty.
Thanks PRR.
Oh, BTW.....I'm using Weber's 25W OT (not the 125ESE shown on the schem....I'll have to change that) & it does weight ~~7 lbs, so I've got a good 14 lbs of iron on this beast.
However, it's in "head" form, so it should come in at well under the 80 lbs total.... :grin:
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I actually kind of like the idea that Merlin has in alternating the bias stages btwn hot & cold to produce cut-off & grid current limiting generating sustain & harmonic complexity along the way. Are there some amp examples w/ soundbites for this kind of design & tone to hear as a comparrison?
Is this (harmonic content) why geezer & others are going w/ SE amp designs for this dumble idea? If so, I thought that a clean PA was what contributed to & was sought after for this type of amp emphasizing the preamp gain? What about the issue associated noise inherent w/ SE amps & high gain stages?
Out of the 4 possible valid questions above "thinking out loud" - why do I get two responses that are only centered on the noise issue but nothing else? That seems to appear as defensive behavior ("why my amp isn't going to be noisy...") & who am I to question anything or anyone and the other questions don't even warrant a response? Thanks guys.
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Well, I can not speak for everyone else. I responded to what I had a response to.
I am not familiar with Merlin's comments/design?
I don't know of any soundbits because I don't know what this is?
I am not aware of harmonic content being associated with SE over other designs?
I was not familiar with the noise with SE amps, so I asked you what you knew?
I thought DaGeezer's answer that his SE amps are quiet was encouraging and respectful. I owned a VibroChamp and a Princeton Reverb at the same time. They seemed equally quiet and/or noisey to me.
You seem to be more familiar & knowledgeable with these subjects than I am. There was no intent to disregard your questions or be disrespectful. I am simply not knowledgeable enough to answer your questions.
The other moderators know far more than I do about this stuff. I have had virtually NO formal training in electronics ever. Strictly a hobbyist learning from the forum and experience.
With respect, Tubenit
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As for me, I understood what you were commenting on, but don't know of any soundclips, so I figured I'd let Merlin field that in case he did know of some.
Regarding why some use SE power amps, I figured I wouldn't presume to know why they choose one topology over another. If I had to guess, I'd think it has more to do with the output power level they're shooting for, or that they're really fond of SE output stages and select them on that basis.
As for whether a clean output stage is a requirement, I don't think it is. However, the original Dumble logic appears to me to be that the preamp distortion is being made with a quite high level of control over various aspects of the distortion; when you do that, you don't really look for output stage distortion. Further, you might view the purpose of the output stage as being simply a way to accurately reproduce what's happening in the preamp up to some high level of volume. But I don't see anything in the design that dictates that the output stage must remain clean, so if someone chooses to use a SE stage, they're only limitiation is the output power their amp will be capable of. A lot of folks here play mostly at home and don't need huge volume, so it seems a workable solution.
As for noise in high gain amps and SE power amps, they're 2 different kinds of noise. The SE output stage only contributes noise when there is insufficient filtering, and Geezer gave the solution: cap to ground, dropping resistor or choke, cap to ground, and then the plate feed. That gives enough filtering to knock down hum without resorting to old Fender Champ tricks of having a bass response too poor to really reproduce a 60Hz signal. It's the route people go when they decide they want to use a good OT and speaker, and then suddenly hear hum.
As for high gain, all high gain amps will have some amount of noise. Or you could better say that there is some limit to how much gain you can have before noise becomes an issue. From what I've read, using high-current or high-Gm tubes early could help minimize noise, but the resistances we use probably account for more noise in amps than anything else. Regardless, I don't build high gain amps much, and I'd defer to soemone who does to share practical methods for insuring low noise.
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I responded to what I had a response to.
I am not familiar with Merlin's comments/design?
You seem to be more familiar & knowledgeable with these subjects than I am. There was no intent to disregard your questions or be disrespectful. I am simply not knowledgeable enough to answer your questions.
Same here...... I just skimmed over the post (as I do most posts these days....over-worked & using every spare minute to get my new build going) & responded to what I quickly had an answer to & understood.
G
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Well, SE amps are by nature Class A which supposedly give even order harmonics whereas Class AB PP supposedly gives more odd order harmonics
Some seem to think output tube distortion is the holy grail. Others get the sound they want with preamp distortion with a clean power amp, as evidenced by the popularity and mystique of the dumble amps and their clones.
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Well, SE amps are by nature Class A which supposedly give even order harmonics whereas Class AB PP supposedly gives more odd order harmonics.
But there are flaws in that logic.
Ready for something that will mess with your mind? If a sine wave has even harmonic distortion (and phase of the various components of the signal are the same), then the waveform will look normal on one side and flattened/fatter on the other side. So "even harmonic distortion" is due to "asymmetric alteration of the wave". The term even comes from mathematics, where the added frequencies are even whole-number multiples of the fundamental frequency, like 2x, 4x, 6x, etc.
Odd harmonic distortion, when all components are in phase, looks like both sides of the waveform are being flattened at the same time. If you added an infinite number of odd harmonics to a sine wave, you'd get a perfect square wave.
Anyway, distortion in tubes happens because the characteristic curves are just that: curves. If they were perfectly straight, evenly-spaced parallel lines, there would be no distortion except for sudden clipping when you drive the plate to B+ or ground. The shape of triode curves are such that when you draw a loadline, you will see that as plate current dips close to zero, the curves bend and bunch together. In this area, a bigger input signal does not result in the same increase of output, due to the bunched curves, as at other points on the loadline. Because this curvature is all on one side of the loadline, triodes typically have predominately even harmonic distortion (flattening of the signal on 1 side).
Pentodes have nearly horizontal curves which bunch together at low plate current. But depending on the size of the load, the loadline may run into a point where all the curves run together at low plate voltage, high plate current. If this is the case, you get odd harmonic distortion (flattening on both sides). Really, you probably get some even harmonic distortion too, because the degree of bunching is not the same on both sides. Flat on both sides = odd harmonics, but not equally flat = even harmonics. How much you have of each is dependant on your load.
You can look up some very complete data sheets, especially the G.E. 6L6GC data sheet, and see a chart that plots how much 2nd harmonic and 3rd harmonic you get for a given setup when the only change is load impedance. Away from 1 critical point, 2nd harmonic is never zero. But one trick sometimes used in the old days of hi-fi was to use a load which minimized 3rd harmonic and maximized 2nd harmonic, then run the tubes in push-pull to cancel the 2nd harmonic. The end result was (hopefully) the least amount of distortion you could manage from those tubes.
So hopefully you'll see that with pentodes and beam power tubes, the characteristic of the distortion can't be assumed; it can vary widely over types and degrees.
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Thanks HPB for the detailed explanation of distortion characteristics which is sort of what I was referring to when I was mentioning the alternating biasing technique. It is creating some very complex harmonic structure through asymmetrical clipping generating new harmonics & textures from raw & biting to rich & creamy. The way each stage is set-up & biased allows the designer/builder to affect the way each stage reacts and sounds be it clean, warm, dirty, gritty, hard, crunchy, etc. (any number of adjectives you'd care to use to describe all of the various content) through a series of stages all biased differently from each other adding up to a unique sound personality particular to that amplifier. Each stage adds in tone, increasing harmonic strcutures, and comlplexity to the next. Along w/ the previous description there's also added benefits which minimizes or eliminates parasitic oscillations, has resistance to blocking distortion, increases sustain and may prevent consecutive warm biased stage's tone from becoming too raw, thin, fizzy or brittle.
This design approach has merit and should be something of interest which warrants attention and experimentation especially in a high gain amp but is contrary to the Dumble philosophy and current schematic the way I see it since he doesn't appear to employ anything like it. Rather a few extra knobs & switches in a tone stack or gain stage doesn't make any major ground breaking sound differences or break throughs in the tone department IMHO. Apparently this way of thinking isn't in the interest of a "let's copy that amp" instant gratification kind of mentality of "the builder" but rather pioneering minds of the true designers out there which the world could use more of.
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Apparently this way of thinking isn't in the interest of a "let's copy that amp" instant gratification kind of mentality of "the builder" but rather pioneering minds of the true designers out there which the world could use more of.
Most of us are just hobbyist, not leaders or pioneers. Still, there's a certain gratification from building an amp even if you didn't think it up. Most of the good stuff has already been thunk of anyhow.
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I'm a leader. Where we going?
Dave
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You could always ditch the power transformer in a large parallel SE amp and use a small switched supply instead. Just a thought anyway. Instead of 20-25 pounds of transformer you could get away with around half that.
If you're going with parallel tubes for SE operation there is some benefit to reduced need for high primary inductance- maybe you could use less turns and have a little smaller OT as a result. Dunno if the increased current and heat of parallel tubes offsets this or not. Come to think of it I don't know very much at all.
jamie
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OK, I've having a problem w/ OT location ......quickly running out of room on my "roomy" chassis.
I have attached a block diagram as to how/where I am placing the OT.....anyone see a better solution?
My concern is the closeness of the 2x trannys & possible interaction, but I can't seem to see any other possible places to put it!
Also, w/ each lump weighing 7lbs+, having 14-15lbs of iron on one end of the chassis isn't real exciting for balance purposes either!
I have also attached the .sch file, so that if anyone has ideas, they can move the stuff around easier.
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I imagine the PT is too big to go inside the chassis, a la a laydown PT but mounted entirely underneath? I was imagining it being inside the chassis, with the OT on the opposite side of the chassis metal. I also envisioned a metal shield inside the chassis between the PT and the rest of the interior, to help block and hum fields. You could route wires through a hole in that shield, using a rubber grommet to protect the wires in the same way that you would in a blackface Fender build.
Apparently this way of thinking isn't in the interest of a "let's copy that amp" instant gratification kind of mentality of "the builder" but rather pioneering minds of the true designers out there which the world could use more of.
Perhaps. It is also a known circuit with known results. The often-overlooked aspect of all this is that the pioneering people may sort out much of the design on paper, but they always breadboard and/or build a prototype to find out how well reality agrees with their theoretical design...
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HBP, no that won't work (PT inside chassis), but what about (if, in fact, problems are present) a shield between the PT & OT?
See attached drawing>>
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Are you sure there would even be problems with no shield, since the cores are shown at 90 degree angles? My suggestion wasn't too concerned about noise (although if you have a steel chassis, it makes a dandy shield) but more about space.
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DaGeezer,
I did exactly what you are considering on an amp build where I used a PT that did not have a cover on/around it. I made a sheet metal shield and it worked just fine having the PT and OT very close to one another.
With respect, Tubenit
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My suggestion wasn't too concerned about noise (although if you have a steel chassis, it makes a dandy shield) but more about space.
No........ the space is available, but I was concerned with possible interaction/noise.
The PT is fully enclosed, but the OT is not....it (the core) is wrapped in tape.
I guess I'll put it there & see what happens, then deal with problems (if any, probably none) later. I'm pretty limited to that location.
Thanks for the input, guys!
G
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Hey Geezer do the headfone trick, and try lifting the ot, maybe a cm or 2 in the air, and also put the ot on its side. i found in my build the went away. the difference was night and day. i had to make new brackets for the ot tho =[[
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I hopefully will start my version of this maybe this wkend?
Note red area:
Anyhow, I am more used to paralleled caps then caps in series using switches. Is there another way to do the same mid boost with paralleled caps & is that 4.7M resistor really needed.
Note blue area:
On the trim going into the OD, it looks like this could be somewhat simplified possibly with a 250k pot and the 250p cap and 470k resistor across the outside lugs? Although I am also wondering if the way it is makes it easier to adjust accurately with the smaller 25kl trim pot?
Any thoughts on those? There's a SCH editable schematic if you can see another way of doing the same thing and getting the same tone.
With respect, Tubenit
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>Anyhow, I am more used to paralleled caps then caps in series using switches. Is there another way to do the same mid boost with paralleled caps & is that 4.7M resistor really needed.
I see it as being a 326pF treble bandwidth cap while both are in series then a plain .02 coupling cap when it's switched out. It's fine leaving it that way unless you're usual stack uses more or less than 326pF? The 4.7M resistor seems like too much w/ 1M being plenty.
>On the trim going into the OD, it looks like this could be somewhat simplified possibly with a 250k pot and the 250p cap and 470k resistor across the outside lugs?
Putting that directly across the pot and keeping it off the board is a good idea. It's an unusual divider set-up & I wonder if it was his way to minimize resistor noise or attenuate RF besides being a creative way to knock off additional signal?
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Jojokeo,
Thank you for the reply!
With respect, Tubenit
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Note blue area:
On the trim going into the OD, it looks like this could be somewhat simplified possibly with a 250k pot and the 250p cap and 470k resistor across the outside lugs? Although I am also wondering if the way it is makes it easier to adjust accurately with the smaller 25kl trim pot?
The way I understood it from dogear's explanations on the ampgarage it is a low gain network designed for the amp to be run in preamp boost mode. With a 250kA gain pot it would not be low gain anymore.
Cheers Stephan
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With a 250kA gain pot it would not be low gain anymore.
I decided I would build it as originally designed with the exception of a smaller filter cap on last B+ node. Probably will use a 16uf or 22uf.
I see a 4.7k to ground + 25k trim pot and a 220k resistor on the non ground terminal. That's 249.7k which is pretty close to 250kA. I'm not sure how substituting a 250k pot would make it a higher gain if the 250k pot were dialed down with very little resistance to ground?
Anyhow, I do plan to build it as originally shown.
With respect, Tubenit
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Got the board built today ........... really looking forward to plugging in and playing this thing about 3-4 wks out!
With respect, Tubenit
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I should be fire'n mine up this week...almost got all the soldering done.
I would be done tomorrow, but for going to the amusement park with the kids & grandkids. Gotta' make sure to take my Excedrin & Dramamine.... :icon_puke_l: :icon_puke_r:
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Oddly, my biggest concern is that the effects loop is not really practical or functional. It's a good idea to have it at the end of the preamp chain to capture the sound created by the preamp, but needs to be knocked down to about 1/200th the size (guessing 20v to about 100mV) for feeding into pedals meant to gulp guitar-signals, then will likely need to be boosted back up to drive the output tube.
OK, let's say for arguements sake ............ HBP (who is my amp building mentor) knows more than I do and that he's correct (just for arguements sake) and that a passive effects loop might not work with a pedal (you know for arguements sake ....... let's pretend he is right) ................... Not that anything like that has actually happened but for arguements sake .............
:wink: :laugh:
Could an active effects loop like is drawn here work correctly????
AND would there be a change in the tone of the amp when it is engaged without a pedal in the loop. Assuming shorted jacks on send and return that are connected.
Any comments about this, please? with respect, Tubenit
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Hello, I'm curious why you feel the need for an effects loop w/ this kind of amp? For driving what effects? You'd be diminishing your tone that you've worked so hard for to colour it down w/ little signal destroying solid state op amps and transistors? :huh: Just kidding you, but that's not the intent for this type of amp and why you're building it, is it? (I wouldn't be thinking while drawing the schematic, "I want to build a cascading tone monster so that I can run all kinds of effects through it!")
More questions that come to mind is how often would you actually have these in use? Do you play w/ pedals a lot through the send & returns in your other amps often enough to justify it w/ this one? What type of effects are you going to be using that are most beneficial to be placed after the preamp distortion that you wouldn't want inserted through the input? Reverb, delays, chorus, phlangers, etc... you'd want for this but I still think that you will definately change your original tone of the amp - kind of comes w/ the territory, IMHO.
Your switched out loop shouldn't affect anything. Switched in (using a jumper?) you might like the extra two gain stages w/out any of the effects? :undecided:
Having a line out is definately something that is practical and useful in many ways as I'm sure you know, wouldn't color down your tone and wouldn't necessitate the extra tube, controls & parts. (unless you need to drive your signal over long cable runs?) But, I would bet :wink: that you will forge ahead and install these if for nothing else, your experimental need to do it anyway. Good luck T I hope it turns out well.
ps - I'm getting ready to drill the chassis for my hot/cold designed stages I described earlier using a load line plotter for each stage. I'm really curious how much I can advantageously push the grid current limiting and cut-off aspects of the design? I didn't copy the Bumble design or values to attain this but it turned out similar flow chart wise in that it goes through the first two stages and switches in another two stages sort of Bumble-like, then goes into a PA like an 18 watt.
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Hey, just wanted to chime in here. I built one like this about a month before this thread got started. I used the tweaked light version for the preamp.
The passive effects loop does not work well with pedals if you drive the preamp hard. It was intended to be used with a Dumbleator (a tube based effects loop). I built one of those too. It does work and actually adds to the tone of the amp even without an effect in the loop.
I am working on a parallel version now.
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More questions that come to mind is how often would you actually have these in use? Do you play w/ pedals a lot through the send & returns in your other amps often enough to justify it w/ this one? What type of effects are you going to be using
I always use a Boss digital delay playing with the band. And I have built in reverb for 98% of the amps I've built & always use the reverb (no off switch to it). Occasionally I use a homebuilt tube screamer for leads. I have a wah pedal that collects dust. Don't own any other pedals at this point.
I'm not real keen on playing amps with no verb or delay. Just a personal preference.
The digital delay pedal works perfectly in every passive effects loop I've used including the one on the T-Lite 5879OD. No issues & nice tone to my ears.
With respect, Tubenit
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OK, what about a paralleled effects loop with wet/dry mix instead of series effects loop? Perhaps that would eliminate a change in tone due to increased gain?
Would this work?
With respect, Tubenit
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Tubenit, I would use the 4.7M if you have one. 1M is going to load the tone circuit more. 4.7M is roughly an order of magnitude higher than the tone controls and should stay out of the way better.
Just curious, where did the circuit inside the blue originate? Particularly the cap and resistor to ground before the OD entrance. I've not seen it on the various D schematics at the other site.
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Scott,
Thanks for the feedback & I'll use a 4.7M. The blue was in the design that Mat on AmpGarage came up with on his mini-HRM.
I probably will change the blue area and get rid of the 470k & 250p to ground.
Anybody out there successfully use an effects loop on a SE design?
With respect, Tubenit
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Well, I gotta admit up front my ignorance on effects loops. It's simply that I've never had one in any amp that I owned or built. So if some of the guys here have a loop they've built and tested, I'd give it a shot.
On the face of it, both of the active loops look good. Again, take the word of someone who's built and used it before taking mine. But there should be some good active circuits out there that take a preamp signal, knock it down to pedal-level, and then reamplify the returning signal to drive the output stage. I have a suspicion that these usually go between the preamp and the driver stage/phase inverter to minimize how much heavy-lifting the return gain stage needs to do.
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OK, my amp is up & running, but there are problems.......
The output volume is very weak, not nearly as loud as 2x 5881's should be (I've built several parallel SE amps, so I know).
1st thing I did to was remove the 180k grid stopper on the power tube inputs (Mat had a 280k on his EL84 version) leaving just the 1.5k stoppers in place. That helped a good bit, but it's still anemic & weak.
So, to verify the preamp is OK, I ran from the effects send/preamp out of this amp, to the effects return of my PP EL84 version HoSo56 (just using the LTPI & power tubes of the HoSo).
It sounds fantastic, very similar to Mat's clips! The preamp is good.
So, I don't have enough signal (voltage swing?) to drive the 5881's :angry:
What can I do? Add a 12AU7 as a driver tube?? That would "kill 2 birds with one stone" I guess, as it would also serve as a return buffer for the effects loop.
Any insight/ideas would be greatly appreciated......
Geezer
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VERY VERY encouraging news that you're getting that same tone that Mat got with the preamp !!! Hooray !!!
OK, here is what I am thinking .............. Mat's min-HRM with a SE EL84 was loud enough that he played it with a band.
And a 5881 is louder than an EL84, correct?
I am thinking that maybe the master vol on the clean OR the last OD pot is acting like a dropping resistor sending too much signal to ground to adequately drive the 5881. So use the 280k as a mixing resistor and add another 1M resistor for the grid resistor. The mixing resistor helps seperate the 1M from the pots.
I'm thinking that may boost the 5881's to be pretty loud and adding the 1M grid resistor using the 280k mixing resistor is a quick experiment to see if my guess is correct?
Worth a try??
With respect, Tubenit
Hey, a favor (if you have time and interest) ........ can you plug the mini-HRM preamp into the 6BM8 HoSo poweramp and let me know how it sounds? No rush but I am very curious as to whether that would work for me?
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Hey Geezer,
Can you run your mini-HRM preamp into the Super SE poweramp to see if there is more volume?
Also I'd try the HoSo56 preamp into the mini-HRM poweramp just to see what happens?
I sure would be interested in hearing how the mini-HRM preamp sounds thru 6BM8 HoSo56 poweramp.
Remember Bruspeed did a version of a Champ D'Lite?? If you look at post #70 (second page of thread), he comments that the amp is really loud using a 6V6. So something seems wrong with not enough volume with two 5881's?
http://www.el34world.com/Forum/index.php?topic=7950.50
With respect, Tubenit
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I've already tried a 1M ground ref from the 5881 grids.....no improvement.
I won't be able to try the different combinations until tomorrow (Friday), but will be glad to do so.
Any other ideas on where to look?
Again (for those just tuning in), the preamp is fine, but there doesn't seem to be enough grunt to drive the 5881's.
I'll post some voltages around the power section as soon as I can......
G
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Did you try a mixing resistor between the 1M grid resistor and the pots?
If you have a pot with 100k to ground & 1M resistor to ground then you'd have 90k to ground and be bleeding off alot of signal (for example).
I have a VibroChamp that I thought was pretty loud using a 10" speaker & Bruspeed's Champ D'Lite was reportedly loud. And your Super SE is pretty loud.
And Mat played his mini-HRM with a band so it must've had some volume to it?
So what's different in this amp's poweramp compared to those others?
And the reason I mentioned trying the Super SE power amp with min-HRM preamp is that might tell you that the mini-HRM preamp does have enough power and the mini-HRM power amp section is possibly the problem.
I also wondered about trying your amp with a single 6V6 to see what happens since it's an easier tube to drive than the 6L6?
The only other thing I can think of right off besides a wiring or component value that's incorrect might be if the OT wires need to be reversed? I built an amp maybe 1-2 yrs ago and posted about the problems. It turned out that it was oscillating without squealing and reversing the OT wiring fixed it. But that might not be applicable with your SE & no negative feedback?
Any chance you have a bleeding resistor on the power rail that has the wrong value?
I am confidant you'll get it fixed and sounding good. Gotta be something rather simple, I'd think?
With respect, Tubenit
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Yeah, the problem right now is that I only have a few minutes at a time to work on it (very busy), but tomorrow I will have more time free to do an in-depth investigation........
G
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Yeah, the problem right now is that I only have a few minutes at a time to work on it (very busy), but tomorrow I will have more time free to do an in-depth investigation........
What !!!! You are taking your real life job and career waaaayyyyy tooo seriously! This is outrageous! You're gonna keep us waiting til tomorrow? Boy, talk about a Hoffman forum cliffhanger. Gee whiz! The suspense is killing me.
:wink: :laugh:
With respect, Tubenit
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Nit,
Some of us have REAL jobs and don't have time to be posting at...for example, 12:35 in the middle of the DAY!!!!! Oh wait.... :angel
Jim
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So, I don't have enough signal (voltage swing?) to drive the 5881's
What can I do?
Measure and think!
What is the bias voltage on your output tubes? What is the measured output voltage when you send that preamp signal out to the other amp? Please note if your meter reading of the signal level is peak or RMS. If you were able to apply a steady sine-wave signal, that would be great, but you might also have a meter that can save an average reading (some Fluke and other DMMs have a button that records and displays maximum peak, minimum, and average voltages for a certain span of time).
My fear is that the preamp generates a big enough signal for driving EL84's (easy to drive), but can't muster enough output for 6L6's and their large(r) bias voltage.
One other off-the-wall idea: have you tried not using the jacks at all, but simply running a wire from the switch to the output tube grids? Sometimes a wiring error on a shorting jack, or just getting carried away with using shorting jacks will lead to problems.
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My fear is that the preamp generates a big enough signal for driving EL84's (easy to drive), but can't muster enough output for 6L6's and their large(r) bias voltage.
I thought about that also ......... but DaGeezer built the Super SE and it worked. And other guys have built a 6L6 SE amp on the forum? So what would be different?
With respect, Tubenit
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One other off-the-wall idea: have you tried not using the jacks at all, but simply running a wire from the switch to the output tube grids? Sometimes a wiring error on a shorting jack, or just getting carried away with using shorting jacks will lead to problems.
The same thought occurred to me today.......I'll try a jumper straight to the grids & see if there is any improvement.
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Well, I'm stumped!
Running the preamp into the 2xEL84 PP poweramp yields a very loud (& absolutely fantastic sounding) output.
Running the same preamp into the 2x6L6GC (or 5881 or 6V6, doesn't matter) poweramp is weak & anemic........the EL84's are at least 3 times louder!
I swapped out the OT for another known good OT......no change.
I ran the output of the preamp directly to the input(s) of the output tubes.....same results.
Triple & quadruple checked the power amp wiring...all looks good.
Plates & screens are @~~360v, cathodes (each with it's own separate 300Ω resistor) are @~~24v.
I'll look more tomorrow, but tonite I'm whipped! :sad:
Geezer
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Plates & screens are @~~360v, cathodes (each with it's own separate 300Ω resistor) are @~~24v.
So your bias is about 24v.
I don't know that Tubenit's first schematic is representative of the original circuit, but I took it on faith that it was and arrived at a guess.
The first schematic showed 270v B+, and a 130 ohm cathode resistor for an EL84. 12w/270v = 44mA, and guess a couple extra milliamps of screen current; how 'bout 50mA of cathode current? 130 ohms * 0.050A = 6.5v on the cathode.
We don't know that the EL84 in the original design ran at 6.5v on the cathode, but 6-8v is believeable in SE. If it's 8v, then 7-8v peak would drive the EL84 to full output power. You have 24v of bias on your output tubes. Therefore, you need 23-24v of peak signal to drive your output tubes to full power. So you potentially have 1/3 the needed drive voltage!
Cut 'n Paste can cause headaches.
But you didn't give a signal level measurement, so we don't really know of the driving signal is deficient. You really need to measure that.
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Running the preamp into the 2xEL84 PP poweramp yields a very loud (& absolutely fantastic sounding) output.
Well then, THAT could be a working solution. :smiley: :wink:
Cheers Stephan
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you didn't give a signal level measurement, so we don't really know of the driving signal is deficient. You really need to measure that.
I'll get that measurement later this morning.
Running the preamp into the 2xEL84 PP poweramp yields a very loud (& absolutely fantastic sounding) output.
Well then, THAT could be a working solution. :smiley: :wink:
Yes, but I'm trying to avoid adding the tube for the LTPI (& all the associated circuitry). However, that may be where I end up......
G
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From my experience building one... the preamp (especially the clean) does not have enough drive. It's funny how you jumped it over to a PP EL84 power amp. I did the exact thing when I first powered the amp up and noticed the low output. AND it did sound great.
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I still don't understand why the Super SE would work & this sounds anemic?
Would it work out for you to have a dual single ended EL84? Maybe just "cap" the 8pin holes with sheet metal and repunch for 9pin EL84's?
Man sorry to hear it's not working out with the 6L6's? Maybe the answer is a buffered effects loop in series using a 12AU7?
Not sure what I'm going to do now? I got a brand new SE OT. :sad:
I might consider a dual SE 6BM8 amp since I'm not real keen on EL84's?
With respect, Tubenit
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...poweramp is weak & anemic...
Can the 5881 amplifier be driven to overload with its own preamp, or does it remain clean until the preamp runs out of gain? If it can be overdriven, then the problem may be in the output stage - although you seem to have covered all the possibilities there. On the other hand, if the output remains weak but still undistorted even with the preamp driven hard, then an overall lack of gain may be the cause.
Do you have another amplifier with a buffered line out that could be used to drive the line in of the 5881 amp a bit harder, to verify that it does have adequate output when driven to its maximum?
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OK, here's some actual voltages.......
Plates = 380v
Screens = 380v
Cathode = 23.5v Each tube has separate Rk, both read ~~the same)
G1 voltage (peak) = ~~6v avg (if I crank everything up, full blast, & hit power chords, I can get a max voltage of ~~17v....the amp is pretty loud at that point, but is way too distorted/ugly sounding)
I guess that G1 voltage is the problem, so I guess I'm either going to have to install a driver stage, or go PP.
Ideas, opinions?
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How about a single triode type (7-pin) as a driver?
I have some nice:
6C4 - http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6C4.pdf
6AT6 - http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AT6.pdf
6AV6 - http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AV6.pdf
I'm thinking I've seen the 6C4's used as drivers (?)
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An extra gain stage before the 5881's is probably necessary. The original push-pull design from which this is derived would ( presumably ) have had some extra gain provided by a long tail pair phase inverter. A relatively low gain single triode section is all that should be necessary. This could be half a twin triode ( eg 12AU7 ), or a single triode as Geezer has already suggested.
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I am thinking if you used a 12AU7 in series you could do a buffered effects loop that might work? I would be tempted to try that if sticking with the 5881 dual SE idea.
Another idea would be to parallel a 12A_7 tube if component space on the layout board is an issue.
With respect, Tubenit
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An extra gain stage before the 5881's is probably necessary.
I tacked in a 6C4 triode & it worked.....it's louder, but doesn't sound completely *good*. I think the 5881 grids are now being pushed too hard.
On the 6C4, I have a 100k to the plate from node "C".
On the cathode, I tried 1.5k, 3.3k & 10k Rk's, both bypassed (2.2uf) & unbypassed.
I tried both .02 & .001 coupling caps to the 5881 grids.
Any suggestions on values to try, or any tricks to get it to behave?
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Maybe a large grid resistorfor the 5881? Do you have a resistor substitution box? What about a trimmer pot as both a grid resistor and sending some signal to ground (5881)?
or what about a trimmer pot going into the 6C4?
Are you using the 220uf cathode cap for the 5881's? Maybe a smaller value like
47uf or 22uf might help the tone?
Sounds like you continue to make progress blazing this trail to your tone!
With respect, Tubenit
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A couple of things come to mind to get the drive level needed.
You already have 2 gain stages for clean and 4 for dirty, so it seems like you should be able to get all the drive you need with the existing preamp.
Tubenit has a schematic on the first page of this thread with your name on it; does the schematic accurately reflect what you have? Supply and plate voltage are likely different throughout... so what do you have?
I think this is the moment where the "design with theory" method will beat the "design by ear" method, until the issue of driving the output tubes is solved. Once that's licked, design by ear has to take over again to make sure the resulting sound is enjoyable.
Also, there's a vloume trim pot of some kind between every gain stage. Until you've played with each of those to know where you like them set to shape the signal through the preamp, giving a good idea of the amount of amplification the last stage will need to make, it will be tricky to know what to change.
It will take me a few days to ponder and do some math (because I'll only be able to work on it a little at a time), but I can probably give some good suggestions on how to proceed if I know that Tubenit's schematic shows the parts values you used, what the power supply node voltages are, and maybe also the plate/cathode voltages for each stage. If you also want to strum your guitar and measure the average signal output of your insturment (peak or RMS), that'll prevent errors from me guessing an input voltage.
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HBP,
Always appreciate you weighing in on these projects! Thanks! DaGeezer and I have been communicating some off line and he confirmed this schematic reflects his current efforts (as of yesterday) prior to the 6C4 being added.
I am wondering IF adding an effects loop would boost things enough to drive the 6L6?? Any thoughts about that?
With respect, Tubenit
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DaGeezer and I have been communicating some off line and he confirmed this schematic reflects his current efforts (as of yesterday) prior to the 6C4 being added.
Sweet! I can infer the supply node voltages from the voltages that are given. It would be helpful to know what a "guitar-level signal" really looks like, instead of just guessing 100mV like we always do.
I am wondering IF adding an effects loop would boost things enough to drive the 6L6?? Any thoughts about that?
Well, let's figure a 12AX7 will have a gain of about 50-60, after considering the effect of plate load resistance and the following grid reference resistor. Let's also figure a guitar can make a 100mV peak signal (maybe his guitar will do more or less, which is why a real-world measurement would be helpful).
2 stages at a gain of 60 each have a total gain of 3600 if all controls are full up and we ignore the tone circuit. 0.1v * 3600 = 360v peak. Except that it doesn't... the supply voltage is close to 360v, and the tubes have to have voltage across them to amplify, so we must wind up at a voltage driving the output tubes well below 360v peak.
However, he has said that his bias voltage is 23.5v (call it 24v), and if you drive the grid so that the grid-to-cathode voltage momentarily reaches -1v or 0v, you will start drawing grid current and have gotten all the clean power the tube is capable of in the particular circuit. We don't need 360v peak, we only need 22.5v peak, or a bit more. Again, we'll use 24v peak as a handy number.
Again, if each stage had a gain of 50, then 24v / 50 = 0.48v peak needed to drive the last stage before the output tubes (whether clean channel or dirty channel), and 0.48v / 50 = 0.0096v peak 2 stages prior to the output tubes if there was no loss between stages. That's only 9.6mV needed to drive the clean channel!
Except... We do have tone circuits and we do have volume pots. So the question is more about are the right supply voltages available and the right operating points chosen to enable each stage to contrinute its effect, but also to drive the output tubes. We know it works fine with EL84's, but they have small bias voltages and only need a driving signal maybe 1/3-1/4 that of a 6L6. I'm just guessing a circuit tweaked to worked great with EL84's without slamming them, now needs to be tweaked a different way to drive 6L6's.
I am hesitant to add more gain stages to this amp, because we know from other 6L6 amps that they shouldn't be needed, and I would be concerned about oscillation.
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I just took readings & updated any component changes.
Here's the actual amp as it stands now:
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Just a thought and perhaps headed down the wrong road ............
The D_mble amp has most of the overdrive in the preamp and the power amp is relatively clean .........correct?
Any chance to use an opamp to boost the preamp into the poweramp? Maybe that would be a clean boost meaning not adding tone or color to the preamp and maybe the opamp would not be prone to oscillation like a tube might be?
Having said all that, I would have NO idea how to go about it. Nor do I know if that would work? :undecided:
With respect, Tubenit
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This is what I was wondering about as being a possibility?
With respect, Tubenit
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Just add the phase inverter and get it done. :smiley:
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Just add the phase inverter and get it done.
On a single ended amp? :huh: :wink:
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This design already has two power tubes (parallel SE) ... Okay add the PI and change the OT.
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add the PI and change the OT.
Yeah, that's where I'm leaning at this point......I already have everything I need, just need to find the room for the extra tube.
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I think I am going to change the OT and use a pair of 6BM8's with LTPI and try that out. If it's not what I want, I'll just switch over to 6V6's.
I probably will put the SE OT in the Hoffman sales in a wk or so. And then replace it with a P/P OT.
With respect, Tubenit
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add the PI and change the OT.
Yeah, that's where I'm leaning at this point......I already have everything I need, just need to find the room for the extra tube.
just curious, but why push the 5881s so hard? your posted telemetry indicates that Pdiss is approaching 29 watts. could that be the problem? tung-sol datasheets show Pdiss max of 23 watts.
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why push the 5881s so hard?
Hmmmmm, good question!
I actually had 6L6GC's in there (30w tubes) & am just used to pushing them up to the max dis.
I put a pair of 60's TungSol 5881's in & lowered their Pdis to 18w (600Ω Rk per tube).....seems to be a bit better (can't really crank it, kids sleeping).
I lowered the Pdis further to 13w (900Ω Rk per tube) & can't tell much more difference @ low volume, but I'll try some different Rk's tomorrow & see what they sound like @ normal volume.
I also have some JJ 6V6's that I'll try........
Thanks! G
BTW, I've been playing the preamp through the HoSo EL84 PP poweramp & it really smokes! If I can get the 5881 SE PA to sound anywhere close (even at a somewhat lower volume) I'll be happy, but as it is, the pre really sounds bad thru the SE PA. We'll see, when I can crank'r up tomorrow.......
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Given DaGeezer's positive review of the preamp into the HoSo56 poweramp, the lack of success with SE 5881 approach & my desire to have a passive effects loop that works ..............
Here is my Plan B. I am going to use a similar poweramp as my 56T with Mat's preamp. The 56T was loud enough to play with the band I sit in with since they mike the cab & it is a really nice volume for a bedroom amp.
And if that doesn't work .......... plan C will be a cathode biased 6V6 version.
Editable SCH version is here:
http://www.el34world.com/Forum/index.php?topic=9149.msg84134#msg84134
With respect, Tubenit
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Okay, I completed a basic analysis of the mini D-HRM amp.
Bottom line up front: you don't need the added 1M to ground, because the master volume pots perform that function. Adding it just gives another place to load down the signal. The big killer to drive is the V2b gain stage plate load. You're gonna need either a bigger supply voltage for that stage, a higher plate load and/or a smaller cathode resistor.
Working backwards based on a needed drive voltage of 25v, you have a volume control and then the overdrive tone circuit. Rough guess based on the Tone Stack Calculator, call the loss in the tone circuit 10dB. If I didn't screw up the math, a drop of 10dB is the same as dividing your signal by 3.165, so we need at least 25v * 3.165 = ~80v peak from V2b.
I made some guesses and some graphical estimates, and assessed V2b as having an rp of 75.5k, a dynamic load presented by the plate load and tone stack at around 92k, and a resulting gain of about 55. You measured cathode voltage is 1.36v, so a reasonable maximum input voltage is 1.3v peak. 1.3 * 55 = 71.5, and after the tone stack you'll get no more than 71.5 / 3.165 = 22.5v peak. That would be good, except you can never drive the output tubes to distort, and you're likely used to hearing that. And that's assuming you have to overdrive master dimed, which you often want some extra at the top. I think I was even generous in guessing the load imposed by the tone stack, and it is likely more severe, resulting in less gain and output from V2b.
If you must add a tube, add a cathode follower driving the overdrive tone stack.
Quick, no-calculation suggestions: You need more supply voltage at C to get more output voltage. Lower the dropping resistor from 44k to more like 27k. You want about 300v of supply at C. Try raising V2b's plate resistor to 220k, or slightly less. Keep the same cathode resistor, or try a 1.5k.
These changes should get the output swing you need from the overdrive stage, but probably at pretty hefty distortion before the output tubes distort.
V1b should be good, as it just has a master volume after it. With a gain of about 60, 25v peak output only requires less than 1/2 volt peak input.
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Any update on this amp?
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Sorry to say, this project is dead (or at least has taken a major detour).
The preamp sounded really good when run into a PP poweramp, but just couldn't generate enough grunt for the big octals SE.
By "really good", I mean:
*with the OD switched in, it had good tone, & I suspect I could have tweaked it to perfection given enough time, but...
*I guess there's a reason you never hear clips of the "Clean" section of the Dumble amps....it was quite un-inspiring.
I am in the process of re-working the circuit into a HoSo preamp, with the Dumble OD section intact. I'm going to run a 2x6V6 PP poweramp (since the octal sockets are already there), but have to add several more 9-pins for the CF & PI.
Lots of extra work, but I'm confident I can turn this project into something useful in the end.
Thanks again for all the help & advice.
Geezer
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IF it can drive a 6BQ5 or EL84 fine, a big bottle that *might* work would be a 7591. It has a similar gain level. (I won't go as far as to say the charts are the same, I know better that they don't match that much.) and is similar to a 6L6 in some other aspects. I've heard it sounds somewhere between a EL34 and 6L6. (I'll know more when I finally get my project, now over a year old, finished. Largest aspect is a cab for the head, the rest is me finishing up the wiring. It is no mini D-HRM or HoSo though.)
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I am in the process of re-working the circuit into a HoSo preamp, with the Dumble OD section intact. I'm going to run a 2x6V6 PP poweramp
Maybe something like this? However, this has more of a D_mble LTPI vs. HoSo56 LTPI.
With respect, Tubenit
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Yep, that's pretty much what I've drawn up.
I drilled & installed the extra sockets last night, now have to re-load the board & re-wire pretty much everything
:crybaby2:
I should have it up & running this weekend.
The only thing I'm wondering about is the Clean Master.....whether to switch it out (as drawn) when the OD is engaged, OR to leave it in the circuit. I can easily try it both ways by simply moving 1 wire, so I'll try it on-the-fly & see what work/sounds best........
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Looking forward to your new innovative amp & tone review! Thanks for blazing a trail that all the rest of us benefit from. It's appreciated.
Best regards, Tubenit
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Sorry if this is rehashing old info regarding the Dumble style preamp into a SE output.
Loosechange had a nice series of SE 6L6 or EL34 SE builds. D'Lite ODS pre, Marshall Jubilee, Blackface champ, ect.
Maybe it's too late for the builds at hand, but these are nice clean builds with a pertinent theme.
D'lite Pre
http://www.el34world.com/Forum/index.php?topic=9003.0
Marshall Jubilee pre
http://www.el34world.com/Forum/index.php?topic=8494.0
Champ Pre
http://www.el34world.com/Forum/index.php?topic=7862.0
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Loosechange had a nice series of SE 6L6 or EL34 SE builds. D'Lite ODS pre
from page 2 of this thread is LooseChange's quote here:
From my experience building one... the preamp (especially the clean) does not have enough drive. It's funny how you jumped it over to a PP EL84 power amp. I did the exact thing when I first powered the amp up and noticed the low output. AND it did sound great.
It left me wondering whether is D'Lite SE was successful or not successful? Particularly since he later recommended switching to a p/p with LTPI further down on page 2. I am not clear what his results were?
With respect, Tubenit
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Was there more gain to be had in the clip that started this thread? While it was a nice clean tone, it didn't sound like it had a lot of extra balls so to speak? Were there any clips that had more gain than that?
Regards,
Don
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Were there any clips that had more gain than that?
Yes
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I finished my Bumblewatt and my clean AND dirty channels have too much power. I can only barely crack open the master to about "2". I'm still working on dialing things in at this point. Also, I didn't exactly follow Matt or Geezer's schem but it's generally close to the same preamp flow chart idea w/ an 18 watt style pa.