Blues Deluxe to ODS #183 Thread

[JazzGuitarGimp]

I’m pretty sure it’s a layout issue caused by the discrepancy in the relative position of V1 between the HRD and the Blues Deluxe.
There looks to be maybe 1” between the input stage grid terminal and the wiring for the 2nd plate of the overdrive circuit, so 4 cascading CC gain stages totalling maybe >100dB gain with a wide bandwidth. It’s bound to have a low margin of stability.
Moving V1 socket so as to be immediately adjacent to its wiring points on the board is one solution. Putting a screening plate between V1 wiring and the board is another.
A bit of scrap sheet metal could be grounded to the chassis and roughly bodged into place, to temporarily serve as the latter and so prove the hypothesis.

Agreed, and this is why I suggested replacing V1A grid wire to board with shielded cable. If this wire is anywhere near the plate wire of the OD section’s 2nd stage (sorry, I am not able to open the HRD2ODS schematic on my iPad since upgrading to iPadOS. Otherwise, I would have specified V2’s pin number.) It will be very problematic since each of the four stages is an inverting gain stage, the grid of the second clean channel stage is in phase with the plate of the fourth stage. If these wires are in close proximity to one another, positive feedback will most likely be the result. There’s a lot gain between these two points, so when I say close proximity, think “as far apart as one inch, possibly more”.

[norburybrook]

Phil,

Did you use the 183 phase inverter and GNFB resistor values verbatim? I see the 183 wants the GNFB resistor tied to the 8-ohm secondary whereas the BM and Blues Deluxe both specify the 4-ohm secondary. Did you ties it to the 8-ohm secondary? If not, you might want to give that a try as it will decrease the overall gain of the amp.

Cheers,
Lou

Lou,

On the main layout for #183 it's tied to the 4 ohm not 8 ohm.


M

[JazzGuitarGimp]

Phil,

Did you use the 183 phase inverter and GNFB resistor values verbatim? I see the 183 wants the GNFB resistor tied to the 8-ohm secondary whereas the BM and Blues Deluxe both specify the 4-ohm secondary. Did you ties it to the 8-ohm secondary? If not, you might want to give that a try as it will decrease the overall gain of the amp.

Cheers,
Lou

Lou,

On the main layout for #183 it's tied to the 4 ohm not 8 ohm.


M

Hi Marcus,

This is the schematic I referenced; it is in the the first post of The 183 5th Generation EL-34’s (High Plate Skyline) topic in the Dumble Files section.

Lou

Edit: and here is the schematic I forgot to attach....
https://ampgarage.com/forum/download/fi ... &mode=view

[JazzGuitarGimp]

Hi Phil,

The more I think about it, I am thinking it’s odd that the amp does not oscillate with all the controls dimed but with no instrument plugged in. This seems to imply the feedback loop involves the first and third gain stages, rather than the second and fourth stages. The next time you are scoping the amp, can you look at the input to the PI, with all controls dimed and nothing plugged into the input to rule out the this hypothesis?

Thanks,
Lou

[norburybrook]

Phil,

Did you use the 183 phase inverter and GNFB resistor values verbatim? I see the 183 wants the GNFB resistor tied to the 8-ohm secondary whereas the BM and Blues Deluxe both specify the 4-ohm secondary. Did you ties it to the 8-ohm secondary? If not, you might want to give that a try as it will decrease the overall gain of the amp.

Cheers,
Lou

Lou,

On the main layout for #183 it's tied to the 4 ohm not 8 ohm.


M

Hi Marcus,

This is the schematic I referenced; it is in the the first post of The 183 5th Generation EL-34’s (High Plate Skyline) topic in the Dumble Files section.

Lou

Edit: and here is the schematic I forgot to attach....



https://ampgarage.com/forum/download/fi ... &mode=view

OK, yes that does show that.

but the layout Tony did here shows it on the 4 ohm.

183ver11.jpg

#102 and #124 are both attached to the 4 ohm so I suspect that would be the case for the #183 as well , no? I could be wrong of course so it wold be good to get this clarified anyway.


M

[pompeiisneaks]

Hi Phil,

I think it may be advantageous to replace the wire that goes from the board to V1, pin 2 with shielded cable. Ground the shield at one end only.

Also, starting at the input jack, can you tell me the functions of all the switches and pots? And if there is shielded cable used on any of these runs, which ones are they?

Thanks,
Lou

I'll see about getting a shielded wire on the second gain stage.

All of the switches and pots followed the existing faceplate of the blues deluxe. Only naming convention I couldn't 'fit' with the front was the 'reverb' which I used as the Drive Volume

The drive pot is near the clean channel volume pot, but I also used shielded wire for both of the send and return on that pot, only the ground connection is a normal wire.

Basically, I have this as the order

Input Jack, Bright Switch, Volume, PAB Switch, Drive Level, Rock/Jazz SW, Treble, Bass, Mid/Mid boost on push pull pot, Master Volume, Drive(Reverb), Presence.

The runs were all pretty short, but the master vol and drive had to be swapped across one another, but I've tried moving lead dress a bit with those with 0 change in tone. The presence is the longest run.

~Phil

[pompeiisneaks]

I’m pretty sure it’s a layout issue caused by the discrepancy in the relative position of V1 between the HRD and the Blues Deluxe.
There looks to be maybe 1” between the input stage grid terminal and the wiring for the 2nd plate of the overdrive circuit, so 4 cascading CC gain stages totalling maybe >100dB gain with a wide bandwidth. It’s bound to have a low margin of stability.
Moving V1 socket so as to be immediately adjacent to its wiring points on the board is one solution. Putting a screening plate between V1 wiring and the board is another.
A bit of scrap sheet metal could be grounded to the chassis and roughly bodged into place, to temporarily serve as the latter and so prove the hypothesis.

I'll try both that and a shielded wire to see what I get ( a shielded metal plate between V1/V2.) I hesitate to punch another chassis hole with the amp fully assembled, as it lends to major damage possible unless I gut the amp again.

~Phil

[pompeiisneaks]

Phil,

Did you use the 183 phase inverter and GNFB resistor values verbatim? I see the 183 wants the GNFB resistor tied to the 8-ohm secondary whereas the BM and Blues Deluxe both specify the 4-ohm secondary. Did you ties it to the 8-ohm secondary? If not, you might want to give that a try as it will decrease the overall gain of the amp.

Cheers,
Lou

Actually I did mention that earlier in the thread, I used the #183 NFB values verbatim, On top of that it uses EL34's so I'm not sure if that changes things either (EL34's have a different impedance than 5881/6L6 no?)

I am using the 8 ohm tap as that's what's wired up in the Blues Deluxe. I can, and do have a 4 ohm tap but it's not used and shrink tubed for safety down by the power tubes etc.

Do I need to change the NFB resistor? the #183 used a 4.7k but the BM calls for a 100k no? I.e. that's meaning I'm giving it 'more' negative feedback as less of the return signal is being impeded by the signal? I would be happy to adjust that too if needed.

~Phil

[pompeiisneaks]

I’m pretty sure it’s a layout issue caused by the discrepancy in the relative position of V1 between the HRD and the Blues Deluxe.
There looks to be maybe 1” between the input stage grid terminal and the wiring for the 2nd plate of the overdrive circuit, so 4 cascading CC gain stages totalling maybe >100dB gain with a wide bandwidth. It’s bound to have a low margin of stability.
Moving V1 socket so as to be immediately adjacent to its wiring points on the board is one solution. Putting a screening plate between V1 wiring and the board is another.
A bit of scrap sheet metal could be grounded to the chassis and roughly bodged into place, to temporarily serve as the latter and so prove the hypothesis.

Agreed, and this is why I suggested replacing V1A grid wire to board with shielded cable. If this wire is anywhere near the plate wire of the OD section’s 2nd stage (sorry, I am not able to open the HRD2ODS schematic on my iPad since upgrading to iPadOS. Otherwise, I would have specified V2’s pin number.) It will be very problematic since each of the four stages is an inverting gain stage, the grid of the second clean channel stage is in phase with the plate of the fourth stage. If these wires are in close proximity to one another, positive feedback will most likely be the result. There’s a lot gain between these two points, so when I say close proximity, think “as far apart as one inch, possibly more”.

The side that's the first input side is the side closest to the OD tube, per the board layout. I.e. it's got V1B as first grid input adn V1A as the second stage recovery in V1. That V1B grid is the shielded wire. The V1A isn't and I can replace that one, but it's farther away from the OD tube on the opposite side.

If I'm gathering some of the discussion, maybe it would suffice to just move the first stage to V1A as well?

I know the first stage is always the most sensitive?

I'll also look at the scope on the PI with nothing connected and with something connected as well to see what I get. The amp is DEAD quiet with nothing connected at all. Even with OD engaged, but as soon as an instrument is inserted, even if I'm below oscillation there's a noticable hum in the drive side, but not 'horrible' it's pretty much the kind of hum I expect with any driven amp. Clean is very clean even at higher volume settings.

~Phil

[pompeiisneaks]

Okay pdf64's point was 100% correct. I just did an experiment with a small thin piece of metal I had laying around. I connected an alligator clipped lead to ground and to that metal and held it with some insulated pliers. I slowly inserted it between the two and the oscillation disappears. This is at max volume and max drive. (master volume is a bit low, but I turned it up a bit and still no oscillations. )

I think since everything is inside already I have a couple (or both) options to resolve this permanently:

1. put a few l brackets on a piece of metal there that can be mounted to the chassis and create that grounded shield
2. swap the sides of the triode so the first stage isn't right next to the OD tube. I.e. swap the V1a and V1b so that V1a is the first gain stage.

What does the collective think is the best thing to do? ( moving it is still an option, but I'm hesitant to basically gut the whole board so I can get in there with the greenlee punches or?)

~Phil

[rootz]

It occurs to me that rewiring the socket is the simplest solution Phil. It would than be according to the original layout: V1a first, b second. Might just to the trick and stop the oscillations. If not, you can always go ahead with the second solution: shielding. For any next project with the same chassis, the logical thing to do would be punching a new hole for V1 and using the hole where V1 currently is for V2. Gets you short wires.

Must admit that I'm slightly surprised you get oscillations from V1's proximity to the OD components on the board. Never had that, but will keep an eye on placement in all my designs to avoid it. Valuable lessons to be learned here.

[JazzGuitarGimp]

What does the collective think is the best thing to do? ( moving it is still an option, but I'm hesitant to basically gut the whole board so I can get in there with the greenlee punches or?)

~Phil

I think before that can be answered, it would be good to know if the oscillation is occurring from the first stage to the third stage, or from the second stage to the fourth stage. If the oscillation is between the first and third stages, then yes, there should be something to gain by swapping the wiring of the two triodes of V1. If the oscillation is between the second and fourth stages, then I think you are better off leaving V1 wired as it is now.

[rootz]

That sounds logical. However, how would you test if 1 interacts with 3 or 2 interacts with 4?

[JazzGuitarGimp]

Lou,

On the main layout for #183 it's tied to the 4 ohm not 8 ohm.


M

Hi Marcus,

This is the schematic I referenced; it is in the the first post of The 183 5th Generation EL-34’s (High Plate Skyline) topic in the Dumble Files section.

Lou

Edit: and here is the schematic I forgot to attach....



https://ampgarage.com/forum/download/fi ... &mode=view

OK, yes that does show that.

but the layout Tony did here shows it on the 4 ohm.

183ver11.jpg

#102 and #124 are both attached to the 4 ohm so I suspect that would be the case for the #183 as well , no? I could be wrong of course so it wold be good to get this clarified anyway.


M

Ah, okay, gotcha. I am afraid this is due to my own personal bias about layouts vs. schematics. I've been working in the electronics field for over 40 years now. For me, the schematic has always been the first and last word on any design. So much so that I didn't even bother to look at the layout in this case. In fact, when I first signed up here on amp garage seven years ago, and realized people were actually building amps from nothing more than a layout drawing, I was floored. I thought "How can you really know anything about the circuit without a schematic?" But hey, that's just this old geezer, spewing on about his ancient biases!

[JazzGuitarGimp]

That sounds logical. However, how would you test if 1 interacts with 3 or 2 interacts with 4?

First the short answer: By opening up all the controls to 10, with nothing plugged into the guitar input, and observing the PI input with a scope for signs of oscillation. When nothing is plugged in, the first stages grid is held at ground, so it is not possible for that stage to oscillate. If you see oscillation under these conditions, the oscillation is occurring between the second and fourth stages.

I actually think the question has already been answered, but I am having a difficult time believing it. [Apparently, I am from the "show me" state, and never knew it]. My gut feeling on this is the oscillation is occurring between the second and fourth stages. This is based on my own amp-building experience. If this were the case, then grounding the grid of the first stage, by not plugging a cable into the guitar input, would result in the amp still breaking into oscillation - and this is not what I have experienced in my own builds. Phil says there is no oscillation with nothing plugged in. I had asked him to reverify this (because I have never seen it happen this way) by opening all the controls up to 10 and looking for oscillation at the input to the PI with nothing plugged into the input. I was just thinking perhaps there was an oscillation with nothing plugged in, and Phil hadn't noticed it yet. But again, that's just based on my own experience. Also from experience, these oscillations occur way above the range of human hearing. But on the other hand, other strange noises (like sudden excessive hiss or 120Hz hum) as the volume control is increased to the point of oscillation usually show up that will imply an oscillation.