Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hiegdk on February 11, 2024, 08:05:17 pm
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First off, apologies if posting stuff like this here is frowned upon, just let me know and I'll remove the post.
One of my favorite amps is the now discontinued Dr Z Delta 88 (I own 4 of them). It's a dual KT88 with an EF86 based preamp. As I've become more comfortable building and working on amps I decided that I'd like to take a stab at tracing the circuit of this amp so that I can reference it in the future if I have to do repairs. This is my first time attempting to trace an amp circuit and I'd appreciate a sanity check by more experienced hands.
I've never drawn an full amp schematic like this before so if there are any conventions I should be following I'd appreciate a point in the right direction. I tried to replicate the sort of layout I've seen in other examples. Thanks!
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First things that catch my eye:
1) Apparent error(s) in the PI circuit. You have R16 & R17 in parallel - I suspect this is wrong.
2) There's also an issue with R14, it should not be 1M - at least not in that location.
3) You may also be missing a cap (0.47-1.0uF) from one of the PI grids to gnd.
I'd suggest a redo from start with the PI, paying closer attention to the grid & cathode circuits.
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The only thing I see wrong with the PI is R17 should connect to ground. Otherwise the cathodes have no DC path for current to flow and the whole circuit fails.
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I would say that R17 should come in on the ground side of C15.
And you should have a resistor at the bottom of your bias pot.
That's all I see initially.
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The only thing I see wrong with the PI is R17 should connect to ground. Otherwise the cathodes have no DC path for current to flow and the whole circuit fails.
Move the grid end of R17 to gnd? Maybe so.. but 50K seems like a large Rk for an AX7.
Also, I need to amend my 3) to "C14 seems too small, by an order of magnitude. I'd expect 0.47-1.0uF".
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A James would be likely for the tone control. See:
https://www.ampbooks.com/mobile/amp-technology/james-tonestack-analysis/
Yours is drawn differently. It may even have been built "different", it is easy to do and not always fatal to instrument amps (it would be noticed in a Hi-Fi amp). But be sure it is not a typo from your hand.
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The only thing I see wrong with the PI is R17 should connect to ground. Otherwise the cathodes have no DC path for current to flow and the whole circuit fails.
Move the grid end of R17 to gnd? Maybe so.. but 50K seems like a large Rk for an AX7.
Also, I need to amend my 3) to "C14 seems too small, by an order of magnitude. I'd expect 0.47-1.0uF".
47K is very common for PI tail in Voxish EL84 amps. Also .022 is commonly used to ground the unused PI input when not used.
https://el34world.com/charts/Schematics/files/Vox/Vox_ac15.pdf
https://sluckeyamps.com/hammond/hammond_ao39.pdf
Also see the two attached Dr. Z amps.
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47K is very common for PI tail in Voxish EL84 amps. Also .022 is commonly used to ground the unused PI input when not used.
Hmm, OK. I'm used to seeing a balanced phase splitter implemented with 6SN7, using a 0.47-1.0uF on the fallow grid. Must be a hifi thing.
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47K is very common for PI tail in Voxish EL84 amps. Also .022 is commonly used to ground the unused PI input when not used.
Hmm, OK. I'm used to seeing a balanced phase splitter implemented with 6SN7, using a 0.47-1.0uF on the fallow grid. Must be a hifi thing.
I can't recall seeing a 6SN7 used in a LTP PI. Back when 6SN7s were commonly used as a phase inverter it was usually a paraphase or cathodyne inverter, so there was no unused grid. Can you post a schematic or link to an amp using a 6SN7 as a LTP PI?
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Thanks everyone! I appreciate the comments and discussion.
Confirmed that R17 should be connected to the ground side of C15.
R14 does indeed appear to be 1M. Also noticed that R4 in the bias supply was actually 4.7K. There does not appear to be a resister between the bias pot and ground, though there is R6 on the other side of the pot.
I've included a gutshot for reference.
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is the amp on the bench or just a pic?
if it's one the bench take a look at the left kt88 to verify solder on this connection, should look "wetted" like the right
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is the amp on the bench or just a pic?
if it's one the bench take a look at the left kt88 to verify solder on this connection, should look "wetted" like the right
It's on the bench. Hard to see in the photo but there's red lacquer on that connection that makes it look a little dull in the photo. It seems to be a good solder joint under inspection.
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Can you post a schematic or link to an amp using a 6SN7 as a LTP PI?
Here's the input amp & PI from the Eico ST70 PA. This was the first tube amp I ever hacked on. Good friend has owned it for around 30 years now, still works fine.
To the right are 4x 7591s.
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Look closely at the amp. You will find the bias line is really connected to the junction of R5 and R6. Like this...
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Here's the input amp & PI from the Eico ST70 PA.
Hmm... Antique Hifi amp. Guess you haven't looked at many guitar amp schematics. :icon_biggrin:
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Look closely at the amp. You will find the bias line is really connected to the junction of R5 and R6. Like this...
Yep, you are correct! Thanks
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Here's the input amp & PI from the Eico ST70 PA.
Hmm... Antique Hifi amp. Guess you haven't looked at many guitar amp schematics. :icon_biggrin:
Nope, not very many at all. Just not really my wheelhouse, so to speak.
But I'm capable of learning.
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Look closely at the amp. You will find the bias line is really connected to the junction of R5 and R6. Like this...
What was their motivation for using a pi-filter in a bias supply? Were they anticipating a need to draw current from it?
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Look closely at the amp. You will find the bias line is really connected to the junction of R5 and R6. Like this...
What was their motivation for using a pi-filter in a bias supply? Were they anticipating a need to draw current from it?
Better filtering. Many of the top name amps used a pi filter. Leo was the only one too cheap to add the extra RC. Seems a single stage filter was sufficient for a circuit with such a small load.
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Nope, not very many at all. Just not really my wheelhouse, so to speak.
But I'm capable of learning.
Hello WimWalther, and a warm welcome. Many vintage era guitar amps - maybe unlike hifi gear - have a special aura, or even a degree of mysticism about them. Or, I should say, many guitar players hold a special kind of reverence for certain vintage amps. Many of these amps did not use ideal circuit designs, and definitely weren't built with top-shelf, premium parts. From what I've read, sometimes things were just done "wrong" from a strictly electrical engineering point of view (for example, I've read that the Fender 5F6-A's cathode follower is biased wrong for ideal audio operation). But, many of these old amps have a unique sound and tone that has been attributed to these (illogical) oddities. And, a lot of guitar players would never dream of "fixing" them, because that might just be where the magic comes from. So, more often than not, guitar players insist on keeping circuits very close to original, out of fear of losing that Mojo, even if it could be done "better."
David
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Look closely at the amp. You will find the bias line is really connected to the junction of R5 and R6. Like this...
Ahh, there's the missing resistor. It was hiding at the top side of the pot.
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Better filtering. Many of the top name amps used a pi filter. Leo was the only one too cheap to add the extra RC. Seems a single stage filter was sufficient for a circuit with such a small load.
The only appreciable load is the resistive divider itself. Otherwise the tube grids draw a couple uA. DC supplies don't develop much if any ripple until a load is taken off.
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hiegdk, would you mind verifying your tonestack drawing? Not saying it's wrong but I was expecting to see something more like this...
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Following this as it looks like a pretty cool amp.
Can I make a suggestion/request to replace the draft schematics throughout the thread, with the most correct/latest single-version for those of us that would like to save it, once it is sorted?
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Can I make a suggestion/request to replace the draft schematics throughout the thread, with the most correct/latest single-version for those of us that would like to save it, once it is sorted?
I'd like to suggest only one schematic in the top post. As the schematic gets revised, edit the top post and replace the outdated schematic with the revised schematic. Then post a message saying the one schematic has been updated.
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I'd like to suggest only one schematic in the top post. As the schematic gets revised, edit the top post and replace the outdated schematic with the revised schematic. Then post a message saying the one schematic has been updated.
Agreed
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.--.
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hiegdk, would you mind verifying your tonestack drawing? Not saying it's wrong but I was expecting to see something more like this...
Will do this evening, and I'll update the schematic in the top post as well.
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hiegdk, would you mind verifying your tonestack drawing? Not saying it's wrong but I was expecting to see something more like this...
Will do this evening, and I'll update the schematic in the top post as well.
I'm pretty sure your TS schematic is correct. The Route 66 has the same TS but different component values. You show two 225pF caps on your schematic but I think they are really different values. Take a look at the attached pic and verify the values of the three orange caps. Thanks.
PS... Forget about the James TS that I posted earlier. I'm convinced that's not in your amp.
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You were right sluckey, I had the wrong values for C12 and C13. The way they were installed made it awkward to read and I mistook the series label for the value (see photos).
I've updated the schematic attachment in the top post to reflect the changes.
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Thanks! This is what I came up with based on your info. Hopefully I got it right. :icon_biggrin:
EDIT 02/16/2024... Updated schematic and added high resolution pdf schematic.
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.... Hopefully I got it right. :icon_biggrin:
That rang a bell. There is another mangled James stack of some notoriety. The Z and the Traynor are more alike than different, tweaks for the different products.
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Thanks! This is what I came up with based on your info. Hopefully I got it right. :icon_biggrin:
Just did a quick comparison to the amp on my bench and to my inexperienced eyes this looks accurate! Thanks sluckey, you're layout is a lot more legible than mine, I'll use that as a reference for how to approach this sort of thing in the future.
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Your first schematic looks WAY better than my first schematic. Good job!
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There is another mangled James stack of some notoriety. The Z and the Traynor are more alike than different
I've never seen that Traynor schematic. It's spooky how closely the layout of my schematic matches the Traynor. Especially since I struggled to make the Delta 88 become a true James stack! :l2:
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Following this as it looks like a pretty cool amp.
I like the simplicity of this amp. If I needed just one more amp, this would be on the top of my list. This reminds me of the AC-15 Lite with big balls! :l2:
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been lurking on this thread, saved the schematic to it's OWN folder, came to the same conclusion, I'm full on 1880's life, but there's just something about a PP KT88 that says please, please....... :icon_biggrin:
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I've updated the schematic attachment in the top post to reflect the changes.
One more question. What are the values for the three pots?
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saved the schematic to it's OWN folder
I can post a pdf if you prefer. Much bigger and easier on the eyes.
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Following this as it looks like a pretty cool amp.
I like the simplicity of this amp. If I needed just one more amp, this would be on the top of my list. This reminds me of the AC-15 Lite with big balls! :l2:
I own 4 of these for a reason... (not a particularly good one mind you). They sound great - loud - but great.
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I've updated the schematic attachment in the top post to reflect the changes.
One more question. What are the values for the three pots?
Will update with values tomorrow.
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Updated the schematic in the top post. Added the pot values and attempted to edit the layout slightly to more closely match sluckey's version.
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Updated the schematic in the top post.
Thanks. I updated my schematic and also added a high-resolution PDF schematic in reply #30.
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I've been considering building a version of this using KT120s. Could someone smarter than me point out what aspects of the amp would need to be beefed up? I'm assuming both transformers would need to be beefed up. Is that it? If so, how best to go about sizing the transformers?
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what's the "objective"? not LOUD ENOUGH with a pair of 88's? :laugh:
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I've been considering building a version of this using KT120s. Could someone smarter than me point out what aspects of the amp would need to be beefed up? I'm assuming both transformers would need to be beefed up. Is that it? If so, how best to go about sizing the transformers?
Probably need to change the way the output tubes are driven. KT120s would require fairly lowish grid leak resistance to maintain stability- which would mean the impedance bridging between the PI and the KT120s would look different - see setup for Ampeg SVT for example
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what's the "objective"? not LOUD ENOUGH with a pair of 88's? :laugh:
More like "more clean headroom", but effectively, yes. :icon_biggrin:
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More like "more clean headroom", but effectively, yes. :icon_biggrin:
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You've got 4 of them... The PI could probably be reworked for better perceived headroom.