6AQ5 56T

[tubenit]

OK, I tried a 56T with EL84's, 6BM8's and 6AQ5's.

I made the least amount of change possible changing between 6BM8 & 6AQ5. I even used the same 300ohm cathode resistor on both. Only change was using a 10k instead of 8.2k into the power tube grid.

The EL84 was sort of 25%Marshallish/75%Voxish.  The 6BM8 was more like 75%Marshallish/25%Voxish and did the best getting the ZZTop sort of sound with the squawks and sustain.  

I'd have to describe the 6AQ5 as sort of a mix of Fender/Vox/Marshall. The bass and mids are clearer more Fender sounding but still has some the chime/jangly Vox top end (unlike my 6V6 amps). The overdrive cranked sounds more like a Marshall/Fender mix. I've played a number of 6V6 amps and I think the 6AQ5 has more chime than a 6V6 does even though they are supposed to be similar tubes. I think the 6AQ5 is maybe the best of the 3 for the kind of blues I listen to?

All three tubes allow the 56T (my HoSo56 version) to still articulate well and it remains a very very expressive touch sensative amp. I can't say one tube made it a more expressive amp then the other. My least favorite is the EL84 which was brighter with the most chime. I thought the EL84 was the least smooth sounding when overdriven. Cranked, perhaps the 6BM8's are my favorite for playing lead and they would hold a note into feedback. I think the clean tone of the 6AQ5 are warmer and more balanced to my ear & may be the best for rhythm for my tastes.  The 6AQ5 also has a very smooth overdrive that is a little more compressed sounding. However, it has less sustain than the 6BM8 or EL84.

A note on the VVR, it allows me to lower the voltages well below the recommended max voltage of 250v. With the VVR allowing the most voltage, the plates have about 285 volts which is below the 305v on Gibson Scout 6AQ5's.

With respect, Tubenit

[imaradiostar]

Sounds great! I'm glad to hear your experiences. My 6aq5 56t is still mid-rebuild, haven't had a chance to revisit it since I started a thread about it in the fall. I repopulated the board to better match your component values and drilled some holes for different transformers then promptly ceased making progress. I wish mine was back together so I could better compare notes!

What is the primary z and size of the output transformer you're using?

I have reason to suspect that some of the differences we're seeing could be chalked up to the different tubes wanting to see different reflected impedances. I was using 6aq5's with a 7k impedance and the amp seemed to be a little dark. I have a 9k primary z output from a hifi amp that originally used ECL86's in push-pull that might be a better match than the one I was using, hoping to try it when I get it back together. I'm even tempted to build one of these amps with a hammond 125 series output so I can try a bunch of different impedances and maybe even get a little core saturation from one of the smaller outputs.

It's funny that the 6v6 and 6aq5 are supposed to be such similar tubes. One day I'd like to take them apart and see just how different the guts are- I'd bet they're more different than the specs would lead us to believe.

You're using a recent model eminence speaker, right? A twelve IIRC?

jamie

[tubenit]

I'm using a DR type OT that is 6600ohm into 8 ohm. And I'm using a 10" Ragin Cajun Emminence speaker which is a very loud 10" speaker with decent bass to it. Having said that, the amp does NOT sound dark at all to me and still has decent chime to it.

with respect, Tubenit

[snoof]

the 6gk6 is another cool tube to try out.  Nice "small rock amp turned up loud" tube.  relatively cheap as well.

[jojokeo]

OK, I tried a 56T with EL84's, 6BM8's and 6AQ5's.

Great info. I have only one amp w/ 6AQ5s that's a "chassis amp" converted for guitar w/ not much done to it going into it's microphone input. That thing overdrives very sweetly going into great feedback and at a reasonable level. I haven't had a chance to experiment w/ it yet due to other projects deemed more important. BTW, didn't you have to change from a 9 pin socket to a 7 pin for that tube? How did you retro fit that?

Lastly, Now I'm waiting (& wanting) for you to try the 6V6 now?!? 

[tubenit]

Yeah, I changed the 9 pin to 7 pin very easily. Just used the 9 pin holes to bolt on a piece of sheet metal over the holes which had 7 pin holes punched out.  Really simply and it's working fine and reversible if needed.

My "5879 Blues amp" is essentially a modified 56T with 6V6's. It is my favorite amp when I am not playing any of them & reflect on which one I like the best. When I am playing any of my amps ....... "that one" is my favorite.

Sitting in with the band, I probably get the most compliments on tone with the 5879 Blues amp.

With respect, Tubenit

[tubenit]

I'm thinking I am done with the tweaking of this amp. The 6AQ5 tubes will stay in. I like them alot.

I changed the 6AQ5 cathode cap to 18uf & used .02 coupling caps post the LTPI.
I experimented quite a bit with cap values in different places in the amp circuitry, but what the schematic reflects does the best to my ears in maintaining the expressive touch sensitivity of the amp while adding a little more warmth.

I've noticed some posts by others trying to reduce the highs on the HoSo56. I tried lots of different options and found a 180p to ground between the Master vol and the LTPI worked the best. It did not change the expressiveness of the amp but did mute those very high chimey ring leaving plenty of chime left. I did try the 180p between the volume and 2nd gain stage but the amp lost a little touch sensitivity in that spot but not so in front of the LTPI. (for an example of another amp using caps like this, look at the BadCat amps).

I also tried LooseChange's Girth mod for the first time. It's a nice boost and doesn't seem to change the tone of the amp as it boosts it. The PAB method did change the tone when it boosted the volume.

With respect, Tubenit

[jojokeo]

Hello J,

There's a few things different from mine & I wonder if you could comment on why you feel they are needed or how they help:
1 - the "smoothing cap" on plate of CF
2 - The cap/resistor switched at the bottom of tone stack
3 - The "ringing" type of cap 180pf feeding the PI point.
4 - I put a 100pf across the plates of the PI instead of #3 above

*I noticed this brightness issue that bordered on a ringing type of sound also initially especially when I was playing w/ different values for the cathode & tail resistor for the PI. I initially had a slight oscillation barely audible when the vol (post TMB stack) was at 8 O'clock position. When I put a resistor & cap across the vol pot it eliminated this and kept most of the high end on the pot when turned down too. Trying to describe the ringing sound - it was like a high end reverbish kind of thing. Is this what you're describing it as? Did that 180pf take care of that?

[tubenit]

Trying to describe the ringing sound - it was like a high end reverbish kind of thing. Is this what you're describing it as? Did that 180pf take care of that? 

That's a very good way of describing and yes the 180pf took care of that to my satisfaction and still allows a nice chime on the top end. Almost everything else I experimented with would erode the expressiveness at some level, but this didn't to my ears.   However, keep in mine that the 56T actually does have reverb.

The switched cap/resistor is LC's Girth Mod that boosts the volume somewhat.

The smoothing cap on the CF knocks off some really high frequencies from what I can tell.

With respect, Tubenit

[Dave]

Just to chime in.....
I have an 18 watter of sorts that I have been running 6AQ5's in. The 350 volts B+ might be a little high for them, but since I have a boat load of them, what the heck. Anyway, I am a 6AQ5 fan myself. When you puch them hard, they have the smoothest sound I have come accross.

Dave

[tubenit]

6AQ5 are very smooth tubes. I agree with that.

The 350 volts B+ might be a little high for them,

A VVR is an inexpensive way of lowering the voltage and easy solution.

With respect, Tubenit

[jojokeo]

Jeff - Just a quick note on tube sampling w/ the amp today. Subbed a 12at for the 12ax in the CF stage w/ great results if more headroom is desired. What a great tone and so much fun playing.

[imaradiostar]

I finally finished my 6aq5 56t- couldn't find my original thread from last year and this one was timely so I thought I'd chime in. When I first built my 56t amp it wasn't quite right- it had some great crunchy tones but it didn't clean up very well. I had a 7k output TF (too low for 6aq5's at the power level I was going for), PT that was too saggy, too much screen resistor drop, 6au6 instead of 5879 (too much gain), and a few various resistors of the wrong value.

I ended up with really similar values and tubes to Da Geezer's original version with a few exceptions. I used 6aq5's with a 250R cathode r. It idles at 8 watts per tube, plate supply of 290v, screens at 280v, PI and CF around 250v, input pentode around 220v. I might try to add another resistor and cap so the PI and CF will be on different nodes but it sounds pretty good so I dunno if I want to mess. I messed with a few other values too- 1k PI cathode resistor, 36k tail resistor and one of the plate resistors is 82k. I'm going to replace that plate R with a 100k when I get a chance. The only other difference is that I wired up a marshall style presence control using a mouser 24mm pot with a spdt switch on the back. It connects grounds the 5k pot (and PI) at zero and anywhere above zero it functions like a normal early marshall feedback loop- lots of good sounds to be had.

The output TF is a nice 9K:8&16 unit from a Pilot hi-fi that used 6GW8's so it's a decent match for the 6bq5's.

It cleans up much nicer now and I'm happy about it. I haven't had a chance to wire up the cathode resistor bypass cap on the 12ax7 yet. I added a grid stopper on the 12ax7 to help with the blocking I was getting at higher gain settings.

I like the idea of setting this amp for moderate gain and using a delay in the loop but I'm concerned with the high output impedance after the tone stack and the ability to easily produce 5 or 6 volts out of the loop with the volume knob maxed- more if I didn't have the level padded down before the volume knob with an extra resistor. I wasn't worried about it till I A/B'd the sound with and without the loop and with a few different cables of different lengths- a lot of the sound is lost with lesser cables and longer runs.

I'm thinking about making a simple little loop circuit on a perfboard using a pair of jfets running off of DC from the output tube cathodes. I figure I could use a source follower to send the signal and a simple little jfet amp to bring the level back up. Trimpots or back panel pots could be used to make the levels usable with the pedals or rack mounted effects of your choice. Thoughts?

jamie

[tubenit]

Imaradiostar,

I just use a passive effects loop with mine and a Boss digital delay and it sounds very good to me. I don't hear the delay & passive effects loop coloring the tone. Just seems to add delay to my ears.

In fact, all 3 of my amps have the passive effects loop that works just fine with the delay.

With respect, Tubenit

[imaradiostar]

I'm betting it has more to do with individual effects. When I use my H2O pedal the results are great. When I use the delay modeler and a few other different effects the results aren't as good. I'm sorry if it came off like I was complaining- I just wondered if anyone has experienced a tonal change.

If I experiment with anything else I'll share. For now I wanted to say I enjoy this little amp a lot and I'm glad you shared your experiences with it.

jamie

[tubenit]

I'm sorry if it came off like I was complaining

I didn't hear it that way at all.  I suggested the passive effects loop to another forum member one time who was using reverb (IIRC) and it did not work for him at all............    So, on the heels of that bad suggestion to him ........ I learned that there are some other factors that play into having one be successful.

I was simply trying to state that I had success with passive effects loop specifically with Boss delay. I don't know if it would work with any other unit or not.
 

Best regards, Tubenit

[moonbird]

Hello all -

I hope you don't mind me slipping in a couple of questions I have had for a while about these "DaGeezer" series amps. From what I can tell this is the only thread that is still alive at this point. I don't want to dredge up any oldies. 

RE: Little Wing (DaGeezer original) - Want to confirm that if one uses the Weber RVBPT (as shown in the schematic) then a separate 5V supply is needed for the 5Y3 -- is that correct?? The RVBPT that Weber currently sells has no 5V tap it would appear.

RE: 56T - What PT are you using to get the 270V HT for this amp? Also -- Is it possible to move the Reverb send and return entry points "past" the Send/Return so that it is available for use with fx? Would the 100K resistor need to come over too? I am thinking about making the reverb switchable and would like it to be on when other fx are in use.

Thx much for all the effort you two have done to make these design so accessible to the DIY community!

[Geezer]

RE: Little Wing (DaGeezer original) - Want to confirm that if one uses the Weber RVBPT (as shown in the schematic) then a separate 5V supply is needed for the 5Y3 -- is that correct?? The RVBPT that Weber currently sells has no 5V tap it would appear.

That is correct, AND don't forget you have to use a "hybrid" tube/SS bridge recto, implementing 2x SS diodes to form the "rear" section of the bridge and the tube forming the "forward" diodes.

G

[tubenit]

I don't remember for sure what PT it was?  It was either a Hammond or it was a DR replacement that also had an option of 270-0-270?  Check out Hoffman's 18w PT. I would think that would work really well.

Yes, I think you could have the reverb after the passive effects  instead of prior to it by moving the 100k insertion point .  I think after the effects is LooseChange's preference and I know he makes great amps & I like his designs.

With respect, Tubenit

[moonbird]

"hybrid" tube/SS bridge recto

Yes thank you kindly I surely will.

I don't remember reading about this hybrid design ... would you mind reiterating its benefits and/or the goals for its design?? thx very much again.

[Geezer]

The advantage of the "hybrid" bridge is that you get all the benefits of a tube recto (ability to adjust the B+ volts, sag etc by subbing in different tube types).

If you used just SS diodes for the bridge, you "got what you got" as far as voltage (which could be too high for your tubes or circuit).

G

[Fresh_Start]

What OT and primary impedance are you all using for these amps?

tubenit specified DR output trannie with 6K in his build.

However, the "typical operating characteristics" shown on tube data sheets are generic starting points, but noticed that all three tubes - 6BM8, 6AQ5 and EL-84 show around 10K at similar voltages (actually 8K for EL-84 but 10K seems fairly typical for push-pull circuits using those tubes too).

I've got no doubt that tubenit's amp sounds great.  Just trying to learn a little more and thinking about a possible "Little Wing" derivative in the future.

Cheers,

Chip

[Tone Junkie]

Yes I to have been curious about your OT transformer for a pair of  6aq5 Geezer you had stated that you had used a 4K transformer and then shifted it down as 8 ohm sounded better on 16 or 4 I cant remember but either way you said if you were to do 4 tubes again you would use a 2k ot was that correct. and Tubnit which one did you use.
Thanks Bill

[tubenit]

I read this years ago and honestly have never paid much attention to impedence matching other then what sounds good to my ears. Never had an issue with an OT and power tubes.  This is from Kevin O'Connor.

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 awful that such things would be suggested. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.

As stated throughout the TUT-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.

With respect, Tubenit

[Fresh_Start]

tubenit, that's one hell of an answer!  I especially like this part which sounds like something PRR might have written:

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.

So the bottom line is that a relatively beefy OT with 6K primary works for you in this amp.  That's good enough for me!

Chip

[Tone Junkie]

Thanks Guys once again you have given me a very helpfull nugget of information.
Thanks Bill

Time to look for transformers

[Geezer]

Geezer you had stated that you had used a 4K transformer and then shifted it down as 8 ohm sounded better on 16 or 4 I cant remember but either way you said if you were to do 4 tubes again you would use a 2k ot was that correct.

My 4x6AQ5A amp has an "intended" 4k/4-8-16 ohm OT. I like the tone better (more clean power) with an 8 ohm speaker in the 16 ohm tap.

So, THAT TAP now becomes the 8 ohm tap for me.

I then adjust all the other taps & primary accordingly (for my purposes).

Thus, the (original) 4k/4-8-16 ohm OT now becomes a 2k/2-4-8 ohm OT......you simply shift all the taps & primary to match the "known good results" tap, as the relationship is relative.

Using this experience, if building a 2x6AQ5A amp. I would use a 4k/4-8-16 OT, plug the 8 ohm speaker into the 8 ohm tap & hope for (expect) similar results (max clean power).

Depending on the amp, you may like/want different results. My amp generates the OD distortion in the preamp, and I want the clean channel to be as clean as possible, so I want the max available clean from the poweramp.
You may like less headroom. That's why I would suggest a multi-tap secondary so you can pick & choose what works best for you.

G

[Tone Junkie]

Thanks Geezer Max clean headroom is good . What Resister and cap combo did you use when biasing your tubes.
Thanks Bill

[Geezer]

250R/47uF on the quad, plates are @~~ 300v, they are biased pretty cool.

Here's the PA section (attached)

[jojokeo]

Depending on the amp, you may like/want different results. My amp generates the OD distortion in the preamp, and I want the clean channel to be as clean as possible, so I want the max available clean from the poweramp.
You may like less headroom. That's why I would suggest a multi-tap secondary so you can pick & choose what works best for you.

G

Geezer, I've been doing this a lot more over the last year or so. Something about the loading aspect of the transformer more properly giving a better response and ultimately tone and feel. In other words going w/ less pri Z than otherwise would be normally spec'd/used.

"Generating OD distortion in preamp and clean poweramp" - this is something I brought up (but was never elabortated upon or I thought being utilized at that time? (if I'm remembering correctly?)) when we were all first going through the analyzing process starting the whole dumblish amp process starting w/ Matt's SE El84 that you brought in from TAG. But this seems to be a big factor of the dumble design and tone correct?

[Geezer]

"Generating OD distortion in preamp and clean poweramp" - this is something I brought up (but was never elabortated upon or I thought being utilized at that time? (if I'm remembering correctly?) when we were all first going through the analyzing process starting the whole dumblish amp process starting w/ Matt's SE El84 that you brought in from TAG. But this seems to be a big factor of the dumble design and tone correct?

Yes.......I used to try to go for the (so called) "grail" tone of power tube distortion, but I quickly found it to be not very useful for my style & live playing situations (either too much or not enough volume & headroom when I needed it....very little control).

I have settled that I really DO like preamp distortion, as long it is the "good" type, not ratty & over-compressed. I (think) I have learned some tricks to get what I want in the preamp OD, and need the most clean headroom I can get out of the PA (without getting too sterile) to accurately reproduce what I've sculpted in the preamp. I may use some rather "unconventional" techniques to get there, but I care not....the resulting tone is what I'm going for, not whether Leo, or Jim or Howie did or didn't do it that way.

The 6AQ5's, while supposedly being a "little" 6V6, have a much different character in my experience, and are my favorite power tube at this point. A quad of them are quite loud thru an efficient speaker.

Geezer

[Tone Junkie]

Geezer so I can get some peramiters to work within on 6aq5,s what would be used to bias them hot for cap and resister with 300volt b+

[Geezer]

The cap would stay the same.

Lowering the value of the cathode resistor will make the bias "hotter" (increase the cathode current), but you have to calculate plate dissipation with each change (and watch for red-plating) to assure you stay within the Pdiss rating.
Maybe order some 10w 300R, 270R, 250R, 220R, 200R, 180R (for a quad, that is....for a pair, multiply those #'s x2). They are pretty cheap & you'll use them for something at some point.

[Tone Junkie]

Thanks Geezer