Regency Amp: moving the bass peak

[Tyrannocaster]

I got a Regency HF-150 amp made in 1954. This is a cool little 6V6 mono amp with massive, wax potted trannies that weigh a ton. The caps had never been replaced and I went through and changed all the coupling caps (I used .022 where the schem called for .033 but I don't think that's that big a deal) and the cathode bypass caps (all stock values) - the filter caps are still good but they will need to be dealt with later. Since they seem to work fine right now I'm looking at other things.

Anyway, I found a schematic online: http://s31.photobucket.com/albums/c386/secretagentxxx/regency%20hf150/?action=view&current=Scan10001.jpg

This amp has a negative feedback loop and I disconnected it and lo and behold - with the new caps and the feedback gone, it's a GREAT guitar amp, and I really did not expect that! I mean, it sounds VERY good...with only one thing that I would change. The Bass control is designed for HiFi use and it's working a frequency that is (to my ear) at least an octave too low. I'd like to shift it up 8va but I don't know what component is responsible for this action - can somebody identify it for me and suggest a new value?

I've played through old mono hifi amps before and they never sounded very good but this one - boy is it different. Of course, kicking the feedback loop out is what made the most difference, but looking at the schematic I'm still surprised; it sounds good through the magnetic phono input, with lots of gain, but it also sounds very good indeed through the line level input (a 12AU7 and also where the tone controls are). Good breakup, tons of high end, I think it's delightful, really, except for the bass control. And BTW, that control has plenty of effect (way more than I would ever use) but since it boosts such a low frequency I can't make use of it.

The schematic is kind of hard to read since the guy uploaded it sideways, but if somebody could examine that tone stack I'd appreciate it. I have also been interested in the power amp section, comparing it to the tweed Fenders I am familiar with, and it isn't much like them.  And with the distributed load setup on the OT I didn't expect it to sound as good as it does (I thought it would be a lot more sedate - it is NOT), either.

I'd never even heard of these before. They are pretty cool.

[Willabe]

I think it's half of a James/Baxendal?

The guys will know what to try.     



                        Brad     

[Tyrannocaster]

I've never had an amp with a Baxendall circuit; if this is one I like it except for the bass frequency center.

[Willabe]

I've never had an amp with a Baxendall circuit; if this is one I like it except for the bass frequency center.

Look at old Ampeg schemos, B15(?) and others in Dougs lybrary.

Here's a link;

http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_B15N.pdf

Do a search for Duncan's amp pages, good info in there.



                        Brad      

[PRR]

> I found a schematic online

Can't read that.

If you can, read the bass-pot capacitor values, then cut them in half to move up an octave.

[Tyrannocaster]

 

Yep, that's the amp. Who knew that June Cleaver could shred? LOL

I can see that I will have to upload the schem somewhere so it can be viewed more easily as I now have a couple of other questions about the amp. I didn't put it on my Photobucket site because they resize everything to 1068 width and this is wider than that - it's hard enough to read as it is without being reduced so I will probably have to cut it into sections.

The bass caps are .04 and .004. When you refer to "cutting them in half" do you mean using values of .02 and .002? That's certainly simple and I could try it out easily by just paralleling the same values (if I can find anything like them in my box). I certainly have SOMETHING I could sub in temporarily just to see the effect. Thanks!

I downloaded the Duncan Tone Stack Calculator and played around with the Bax control, but I'm not sure that's actually what this is. If it is, it isn't exactly like theirs.

[Willabe]

The bass caps are .04 and .004. When you refer to "cutting them in half" do you mean using values of .02 and .002?

Yes. Smaller value cap = less bass.

I downloaded the Duncan Tone Stack Calculator and played around with the Bax control, but I'm not sure that's actually what this is. If it is, it isn't exactly like theirs.

I think it's only half of it, just the bass controll without the treble half.


                              Brad    

[John]

That's certainly simple and I could try it out easily by just paralleling the same values (if I can find anything like them in my box).

You probably already know, but I'm making  sure- caps work the opposite of resistors. If you parallel them, the capacitance increases.
 As in, a 2uf || 2uf = 4uf.

[Tyrannocaster]

That's certainly simple and I could try it out easily by just paralleling the same values (if I can find anything like them in my box).

You probably already know, but I'm making  sure- caps work the opposite of resistors. If you parallel them, the capacitance increases.
 As in, a 2uf || 2uf = 4uf.

Damn; yes, I did know that and just wasn't thinking. But if you hadn't reminded me I would have gone in and put in something and then scratched my head until I remembered. I've added caps in parallel on power supplies often enough that I shouldn't make such a dumb error. Oh, well...

Thanks.

[sluckey]

I rotated your schematic. Seems legible enough to me. Also attached the Bax tonestack for the B15. Even though the B15 is a bass amp, this exact tonestack is used in several of the Ampeg guitar amps.

[Willabe]

Looking at both schemos it looks like their the same, both Baxendall. It's just that your has the treb/bass controls flipped left to right.

And I think they're both active because the next stage their feeding has no grid return R and that the tone controls are acting as the grid return R, making them active tone controls?

Have I got this right?


              
                               Brad      

[Tyrannocaster]

Sluckey, thanks for flipping that. I guess you must be right; the conventional way of drawing the Baxandall circuit always confuses me with the way it draws the two pots and mine isn't drawn that way. But I'm not sure about what to change if I want to raise the peak of the Bass control's curve - playing with the Bax circuit on the Duncan simulator gives results that never do what I want, at least not so far, anyway. If I have to, I can live with it the way it is, because it is nice for the rare times when you want to play jazz at low volumes and yet it sounds very good cranked all the way...as long as you turn the bass completely down. It's quite Plexi-like when dimed; didn't expect that, either. I thought it would be more tweedy.

Willabe, I think they must be active - there is so much more range available than I'd normally expect otherwise. It's really a very good sounding tone control in many ways; I think it's ideal for its intended purpose - listening to music through a flat response (so-called) system. But all of these components interact and it looks like it's not as simple as just halving the values of the two caps if you want to raise the area the bass affects.

[sluckey]

And I think they're both active because the next stage their feeding has no grid return R and that the tone controls are acting as the grid return R, making them active tone controls?

That does not make them 'active'. Both of those tone stacks are passive.

An active tone control is usually placed in a negative feedback loop to actually control the gain of an amplifier stage. Putting the tone control in the feedback loop varies the gain of the amp and it is frequency sensitive. It's kinda like adding a 'presence' control in a global NFB loop of a power amp in that the amount of feedback changes with frequency, thus affecting the gain of the amp differently for different frequencies.

Here's an example of an active Bax. Notice the feedback path from the plate of V1B back thru the tone network, to the grid of V1B. That's what makes the tone stack active.

[Willabe]

Ok, thanks sluckey, I think I get it/see it now.     

But all of these components interact and it looks like it's not as simple as just halving the values of the two caps if you want to raise the area the bass affects.

PRR said it will, I'd go with what he say's. Change the .004 to .002 and .04 to .02 that are on the bass control. If I understand him right that will raise the -3db bass roll-off an octave up.


                      Brad     

[Tyrannocaster]

Changing to those values didn't do much that I could hear. I took those out and put in .01 and .001 just to see and it's still not much different from the stock configuration, if at all. I'm out of caps to try, and out of ideas.  Thinking that this is just how it's supposed to be, maybe.

[Willabe]

Is it C10 that's feeding the tone stack? A .15 coupling cap? If so try changing that to maybe .02? Then adjust larger or smaller from there.

Also you can make the output tubes cathode bypass cap (250uF?) smaller, try 100uF or 50uF and see what happens. Make sure you have a high enough voltage raiting on the cap and + side to the cathode.


                      Brad      

[Tyrannocaster]

Well, I have messed with the amp enough to know quite a bit about how it performs, now. I find it interesting: the line in section is considerably brighter than the phono section - you can actually use that bass control there IF you are playing at very low volume and you want an ultra mellow, jazzy sound. The mag phone section has more bass and I think I probably need to try to tame that. I don't know how much I should do with the tone circuit given the lack of change I experienced with the alterations I made. I'm thinking that the coupling cap coming from the first triode in the amp could probably be a LOT smaller - that would dump some of the bass from the phone preamp but leave the line inputs alone, which I want to do. That coupling cap that feeds the tone stack is too late in the line - changing it would both circuits in the amp; however, C3 (right after triode #1) is an .05 and I think I could go WAY smaller there. I don't fully understand the switching and all that EQ stuff that the second triode does (that's an understatement) but I don't use any of it anyway and the first triode seems simple enough that even I can grasp how to make simple changes there. 

[sluckey]

Just make the tone stack look exactly like the Ampeg.

[HotBluePlates]

... the line in section is considerably brighter than the phono section - you can actually use that bass control there IF you are playing at very low volume and you want an ultra mellow, jazzy sound. The mag phone section has more bass and I think I probably need to try to tame that. ...

There's the key.

We probably should've noticed this earlier. Any kind of phono stage is going to have EQ heavily tilted towards bass and throwing away highs. When the master disk is originally cut, bass has to be cut heavily and treble boosted. That's because large bass signals would cause large cutting head excursions, and leave the walls of the groove too thin (or try to move the cutting head to the space the next groove has to be). A bonus is that boosting treble on record and cutting it during playback reduces noise.

But there is also some low frequency at which it is usually desirable in a phono amp to cut lows heavily, to reduce rumble caused by a slightly warped record, or you walking in the room.

If you can sort out the switching scheme, rip out everything between V1a and V1b that's not a coupling cap or grid reference resistor. The fixed phono EQ is between those stages. Reduce the V1b grid reference resistor to 1MΩ.

[PRR]

> the Bax tonestack for the B15

James.

Bax is wrapped around a hi-gain tube and uses linear pots. James goes between two stages and uses 10% Audio taper pots.

> as long as you turn the bass completely down

Are you going into the PHONO input?

That's got great gobs of bass boost and treble rolloff (to make-up the rising response of magnetic phono pickup).

Disconnect everything on pin 7 of V1. Put a normal 1Meg to ground and stuff your guitar in there, optionally with a 33K series resistor if you catch radio stations.

That won't be exactly right. You may be loving the treble drop, or the way V1a overload is mellowed by the following falling response after. But easy to try and easy to un-try.

I don't think the Tone Control is very wrong. I think you are mis-using it to negate the excess bass of a PHONO input.

Side-Note: there are at least three different "phono inputs" found on tube gear. Through the 1940s, the 78 era, huge cartridges gave fairly strong flat response, and make acceptable if low-gain guitar inputs. With 33s we got moving-magnet microgroove carts which need heaps of bass boost all across the audio band, generally not good for anything else. In the late 1950s the low-price gear got Crystal needles which are fairly strong and flat, these inputs work on guitar in a low-gain way.

> When the master disk is originally cut, bass has to be cut heavily and treble boosted

Not really true; depends on your point of view; anyway not our job.

The magnetic playback cartridge is Velocity sensitive. For the same size groove-wiggle, higher frequencies play back stronger. Groove wiggle is strictly limited by play-time per side. So the ideal cut has most of the audio band wiggling same-width, but the magnetic playback cart rises 6dB/octave. We un-do this with the generally -6dB/oct "RIAA" response. (For historical reasons there are jogs at 500Hz and 2,150Hz, and at 50Hz.)

[Tyrannocaster]

If you can sort out the switching scheme, rip out everything between V1a and V1b that's not a coupling cap or grid reference resistor. The fixed phono EQ is between those stages. Reduce the V1b grid reference resistor to 1MΩ.

LOL; I'm laughing because of the phrase "If you can sort out the switching scheme", which was the very first thing that occurred to me when I saw the schematic. It's a nightmare for me - and inside the amp, it's all really messy point-to-point, which makes it hard to follow and even harder to alter. I would love to just get rid of all the EQ stuff but the way the switch is wired I think figuring it out and accomplishing it is beyond me at this stage.

With respect to the idea that we're correcting the phono EQ - yes, that's right, but only up to a point. The Bass control has way too much bass available AFAIC and it's too low AFAIC and all of that is tru whether you are using the phono input or the line input. The difference is that it's not as bad with the line input, that's all. So I'm concerned about the bass across the board, not just in the phone preamp. But I can live with it and as I mentioned earlier, that much bass is actually kind of nice when you're playing really quietly and you want a mellow sound.

Oh, I should mention that there's one position on that switch that is labeled "Flat" (even though it is supposedly the phono input) and that's the one I've been plugging into.

[HotBluePlates]

With respect to the idea that we're correcting the phono EQ - yes, that's right, but only up to a point. The Bass control has way too much bass available AFAIC and it's too low AFAIC and all of that is tru whether you are using the phono input or the line input. The difference is that it's not as bad with the line input, that's all. So I'm concerned about the bass across the board, not just in the phone preamp.

Gotcha. Yeah, reducing the size of both caps connected to the bass control by equal ratios is the way to raise the point where the bass control is effective.

But unless you're using the line channel, once you get close the inherent EQ of the phono channel will be misleading.

The B-15 schematic Sluckey posted used 0.01µF and 0.0001µF. I'd consider those your starting point, and you may reduce from there.

Since no one's likely to be collecting a Regency, or you're willing to mod anyway, why not simplfy the wiring a la JCM800? Set up a 1/4" jack that feeds your "line channel". Make it a switching type, like you'd normally use for an input jack, but don't connect the switched lug to ground.

Set up a second 1/4" switching jack, which does have its switched lug connecting to the ground lug. This will feed your phono channel. Output of the coupling cap for the 2nd stage of the phono channel gets connected to the "Line channel" input jack switched lug.

This arrange avoids the switch, gives line-channel gain when you plug into the "low" jack, and full gain when plugging into the phono jack. There is probably a more specialized jack you could use for the line channel, to be certain no noise exists when nothing is plugged in the amp. However, you'll either be playing through the phono channel (noise from unused line channel not an issue) or the line channel (jack in use, noise not an issue).

[Tyrannocaster]

Since no one's likely to be collecting a Regency, or you're willing to mod anyway, why not simplfy the wiring a la JCM800? Set up a 1/4" jack that feeds your "line channel". Make it a switching type, like you'd normally use for an input jack, but don't connect the switched lug to ground.

I like this idea, and something similar had occurred to me; the reason I haven't tried is, quite simply, the maze of the schematic for the switch and the maze of point-to-point scares me off; I'm afraid that without nearly gutting and starting over I will end up with an amp that doesn't work and I won't have the skills to figure out why. That switch is a friggin' nightmare for me, both on paper and physically. It's got stuff attached to it in three dimensions and three layers and it's a case of "To get at THIS first you have to remove THIS" only it's not all stuff that should come out (I think). As it is, right no I have an amp that works with both line in and with hi gain, the only downside being that they do not have exactly the same sound. Yet the hi gain "channel" sounds good cranked and as I mentioned earlier, it's rather Plexi like. The line input is brighter and a hell of a lot cleaner, as you'd expect.

I'm trying to get my mind around the mess that is the switch right now. Part of my problem is that I don't understand what each component of the second triode's circuitry does so it's hard for me to tell what should stay and what to keep. I have the impression that I'm using just the first triode and not most of the circuitry associated with the second (look at the way the first triode goes out right to the switch) but surely SOMETHING is happening with that second triode...only I thought that since I am using the so-called FLAT input I was bypassing that.

Ah, the perils of those who know only enough to be dangerous.

[Tyrannocaster]

HBP, your idea sounds better and better to me as I think about it. But I really need to feel that I understand the second half of the phono preamp. It looks to me like when I am switched to the FLAT mag phono setting the signal goes right out of V1A into the second tube V2A, the 12AU7, bypassing V1B. I can't figure out how to repost the schematic using Sluckey's rotated version, darn it, so you'll have to scroll back up and find it again to look at it.

Can somebody confirm or disprove that this is how the preamp works? I'd just as soon get rid of all the stuff that's not being used when I play through the Flat mag phono input but there's no way I'm going to try to take out that switch until I have a better handle on what's going on here.

[sluckey]

Let me throw out another idea (and it's what I'd do with that amp)...

Gut it all the way to the 6V6 grids. Maybe even redo the UL screen taps. Then you have a P/P cathode biased 6V6 power amp with matching power supply, AND 3 little tube sockets. Your choices are now wide open. Cathode biased Plexi 6V6 is the first thing to pop into my mind (easily convert to fixed bias if you want). Or Princeton with trem, or on and on.

You could leave everything to the right of the volume control if you want that PI. Messing around with modifying the hifi preamp is gonna take a lot of time and you likely will never quite get what you're hoping for.

Here's what I'm talking about. (It may be easier to just take the iron and start with a new chassis.) Just thinking...

[Tyrannocaster]

Sluckey, gutting it and starting over was my first thought when I got this. But when I heard it without its negative feedback loop and heard its Plexi-like phono circuit I got kind of fond of the old amp. The physical layout does not lend itself to using a circuit board at all and that's the kind of amp I'm used to doing; this one was done point to point (which I HATE working on) but it would be easier to do a new amp in it using that technique than trying to put a board in it; I just don't really want to make another point to point amp when I already have one that's a PITA to fix. Doing trial and error experiments on a point to point amp is a nightmare AFAIC. If you wire it the way it really should be done it's nearly impossible to undo what you just did; if you don't do it the way it should be done you're likely to have problems later on.

I LOVE old Fenders because of their easy-to-fix circuit boards. They can be tight in the chassis, but overall they are a work of genius.

[PRR]

> there's one position on that switch that is labeled "Flat"

I see it and do not believe it. It may cut out half of the EQ.

The phono preamp is V1a conventional amp, 470K-10K-0.03u bass EQ, 47K-22K-12K-0.002u treble EQ, V1b conventional amp stage, and out to selector switch and level control.

Whacking-out all that EQ and going V1a to V1b will probably be WAY too much gain, and tend to squeal. That's why I say find V1b grid and go right in there.

> the way the first triode goes out right to the switch

It only goes to the section which shorts-out un-used inputs. When in Phono, it shorts In1 and In2; when in any line it shorts the Phono Input. It's all very hi-Z circuitry so a "live but not switched-in" source would bleed/leak into the desired source.

If the Bass knob action is too dramatic, you want *bigger* caps not smaller.

Also I suspect the grid of V2b should go to the *other* end of the 47K which isolates bass from treble pot wipers. This may be a design mistake, or may be correct in the amp and mis-drawn on the PhotoFact.

[Tyrannocaster]

If the Bass knob action is too dramatic, you want *bigger* caps not smaller.

It's not so much that the action is too dramatic (though it certainly is strong), it's that it's centered too low for my taste.

Also I suspect the grid of V2b should go to the *other* end of the 47K which isolates bass from treble pot wipers. This may be a design mistake, or may be correct in the amp and mis-drawn on the PhotoFact.

Wow, sharp eye there, PRR; you're right. There's no resistor between that tube and the tone treble control's wiper. I never thought bout errors in schematics before; what a potential nightmare.

Thanks for summarizing that first tube for me - it was obviously way beyond my ability to figure it out on my own. I see the logic of simply jumping in at V1B...if I can sort out the switching. I think at this point I will live with the amp for a while and try to decide its ultimate revision fate. It's a neat old amp and I actually really like the clean second section a lot. The distorted first section, while it has a nice really cranked tone, is not as appealing to me so I don't think I would lose that much by changing it. But I probably don't know everything about its behavior yet and I don't want to take a wrecking ball to it quite this soon.