Princeton Reverb Scratch build blocking dist

[2pupwonder]

First post, quick intro. I am a hobby builder who has modified a few Blues Jrs and pedals, and scratch built my own AA1164. I occasionally play guitar.

I build this amp back in early 2017 based on Hoffman's turret layout. The only changes I made are using a Deluxe Reverb OT. When I built it, I noticed it had unpleasant distortion when volume was 3-4 and bass was turned up above 3. At the time I thought this was the 'strange overdrive' everyone attributed to PR's but since then I realize it's blocking distortion and is abnormal. During and since my build, I have learned a lot more about tube amps from various sources - Billm - Hoffman/EL34 world - Rob Robinette - Valve Wizard - Geofex/RG Keen - Hardway Vintage blog etc.

A few notes on my build:

- 1 It follows Hoffman's board layout exactly which includes:
- Power tube Screen Resistors 470 ohm 3w
- Power tube Grid Resistors 1.5K ohm
- Adjustable fixed bias
- 5AR4/GZ34 rectifier

- 2 changes I made are:
- DR OT
- bumped up first filter cap (Node A) to 47uf per a Billm blog post

- 3 I used the following transformers:
- PT - CE Dist P-TF22772 - this transformer seems to have a slightly too low heater current rating, and 2-3amp rating on the rectifier heater (suitable for 5U4GB) This PT is discontinued.
- OT - Hammond 1750H - Deluxe Reverb OT - input impedance 6.6K ohms - output 8 ohms. - typical PR OT's are 8.5K ohm primary impedance. not sure if this matters.

- 4 Per Hoffman's layout, I used a 5AR4/GZ34 rectifier, mine is from JJ c2016 production.

- 5 Power tubes are JJ 6v6S - matched pair - biased (if I remember correctly) to around 18ma - slightly cool

- 6 First power supply node filter caps are separate caps, not a can, values are 47uF for A node, and 22uF for B,C & D nodes. First stage is grounded with HT and bias. Everything else is grounded with the preamp to a buss bar on the back of the control pots.
- 7 Power node voltages at idle are:
- A 431vdc
- B 421vdc
- C 337vdc
- D 255vdc
- I understand these are higher than normal for A & B, but not crazy.
- 8 I have been through the build several times to be sure it's correct. I am pretty sure it is at this point. It works normally at low bass settings and sounds wonderful clean.
- 9 I added a 470K grid stopper to the PI, this helped remove some harshness, but not all.


I have come up with the following solutions but wondered what the relative merits or disadvantages of them are:

- V1 Cathode resistor bypass capacitor changes. Per Hardway vintage blog, I could change V1A cathode bypass cap from the 22uF standard value, down to a 4.7uF to attenuate incoming bass. Some folks say to do this on the first gain stage (V1A) and others say to do it on the second (V1B), Also, some say 10uF often solves the issue and affects the tone less. I understand cutoff frequency for 4.7uf is around 22hz and 10uf is around 10hz, Human hearing cuts off between 12-20hz. Which position is better and which value is adequate? I don't want to change the clean character if I can help it.

- V1 or V2 pin 6 coupling cap - currently .022 per Hoffman's layout. Some say to reduce to .01 (which i have on hand), .0047, or .0022. Is there an advantage doing one versus the other? Any input which value or location would be better?

- Reduce value of tone stack bass cap from .01 to .047 or .022.

- Reduce Power Tube input coupling caps - currently .1 - some sources say this was .01 on Fender vintage PR's tho I only see that on a PR ii Riviera schematic. The AA1164 schematic calls for .1. From research, I believe my issue is pre-PI which this would not help.

- Increase power supply resistors to change C, D, and E node voltages. I don't have values, but could run a sim on this in psud2 to see what the reduction would be.

- Install a 5U4GB or 5Z4 rectifier to drop voltages - my transformer is adequate to handle the extra heater current - would probably need to reduce Node A filter cap to 40UF or below. This is a bit more expense than I was planning.

- Install a 12AT7 in PI position. I understand this does not do much.

Any input on these solutions is appreciated. I like what this community has.

[sluckey]

Reducing both preamp 25µF cathode bypass caps to 4.7µF and reducing both 0.1µF PI output caps to .022µF or .047µF will probably get you to the sound you want.

[2pupwonder]

Thanks Steve, I need to get the parts in house and try that.

In the meantime, I have a .01 cap on hand that I could swap the .022 coupling cap for on the output of V1a. Is there any point in doing this? or is it a waste of time?

[sluckey]

I have a .01 cap on hand that I could swap the .022 coupling cap for on the output of V1a. Is there any point in doing this? or is it a waste of time?

I would leave that cap alone. Replace the other suggested caps first. If still not satisfied, continue experimenting.

[Stevem]

Is this issue you have on both pickups, or only with your neck pickup

[pdf64]

From research, I believe my issue is pre-PI which this would not help.

That seems strange, what sources lead you to that understanding?

[2pupwonder]

Hello, thanks for the helpful responses here. To answer the questions:

Is this issue you have on both pickups, or only with your neck pickup

Yes, this is on all guitars, all pickups. My higher output LP does it slightly lower on the dial than my Tele.

That seems strange, what sources lead you to that understanding?

I probably phrased this incorrectly, based on my understanding, this was my reasoning: the distortion issue occurs within either the PI or Power section, however, the solution would be in the preamp to reduce sub-bass, the preamp circuit is giving the PI and power section more bass than it can handle, possibly magnified by the upgraded OT. Probably more assumptive than anything.
- I understand that the PR preamp uses 1.5K cathode resistors bypassed with 25 or 22 UF capacitors on all preamp stages (this is typical for all Fender Blackface and most Silverface). This gives a cutoff around 5-8 hz, well below human hearing which is very energy intensive to amplify. While this may not cause issues within the preamp itself, the cathodyne phase inverter is susceptible to blocking distortion because of this.
- Another possible issue for this is that the cathodyne is working ok and passing the bass, but because of the upgraded DR OT, the cathodyne is overdriving the power section into hard clipping.
- I don't remember where I read all of this, but it would have been on the sources in my first post.

I'm working on a new amp now, currently in design and part ordering phase. So, wanted to get this one shipshape before diving into the next one. I am continuing to read RG Keen, Valve wizard, and Rob Robinette to deepen my understanding of how these work.

The new amp is based on a Deluxe reverb, but deleting the normal channel and tremolo and packaging it in a Princeton reverb chassis and cabinet.

[2pupwonder]

For interest, this is the gutshot.



I have since made a few grounding changes:

- Green Safety ground is looped and on its own screw near where the power cord comes in.
- HT center tap, A node capacitor, heater hum balance, and bias ground are on their own screw in front of the PT. taken off the transformer screw.

I am considering reworking and modding a bit more, the amp has a bit of hum and hiss. So adjusting lead dress and making some mods:
- 1 rewiring pots and twisting the leads and eliminating the buss bar. I get some occasional thumping from the trem which lead dress should help.
- 2 rewiring reverb driver, and looping the plate connection around the outside of the tube - reverb is intermittently noisy with some hiss.
- 3 removing the cap board and placing the caps in front of the board.
- 4 Grounding tub driver and node B together separately from rest of preamp.
- 5 rewiring heaters with more wire for a higher arch. currently they are flying in about 3/4" above the chassis. Also would clean up the left side of the chassis better, push the heaters farther into the corner where the power cord comes in.
- 6 Adding a 25 Mid pot. possibly with a low limit resistor - 2.2K or so. I would rework the front panel for this, making this like a 'factory install', not a rear panel add-in. This would simplify items 1 & 3.

Based on what you see here, is there benefit to what i mention here?

2P

[xtian]

I see you have made a common error (I did this, too!) at the reverb driver tube. The blue wire to the reverb transformer that connects to pins 1 and 6 must loop AROUND the tube socket, or it will cause ugly distortion (whether you have reverb turned up or not).

https://fenderguru.com/wp-content/uploa ... layout.png

[2pupwonder]

I see you have made a common error (I did this, too!) at the reverb driver tube. The blue wire to the reverb transformer that connects to pins 1 and 6 must loop AROUND the tube socket, or it will cause ugly distortion (whether you have reverb turned up or not).

https://fenderguru.com/wp-content/uploa ... layout.png

Yes, I actually made that change when I did the grounding updates.

[Stevem]

I see your heater feed wires are twisted much tighter then they need be and this can be a problem, but not in regards to distortion.

[2pupwonder]

I see your heater feed wires are twisted much tighter then they need be and this can be a problem, but not in regards to distortion.

Learning something new every day! What issues can it cause? Insulation abrasion and shorting? Or something magnetic? How tight is too tight vs too loose?

2P

[xtian]

I see your heater feed wires are twisted much tighter then they need be and this can be a problem, but not in regards to distortion.

Learning something new every day! What issues can it cause? Insulation abrasion and shorting? Or something magnetic? How tight is too tight vs too loose?

2P

After nearly 100 builds, I have not noticed a hum reduction benefit to twisting heater wires. It does help them stay together for nice lead dress, but close and parallel is just as good. Using too many twists increases the wire used and increases resistance, and can also cause breaks when done as tightly as we see here.

[Adrien]

Hi 2P, I had some similar issues with my Hoffman PR build. It seems like the root cause of that nasty distortion is actually the reverb driver being overdriven at higher volume levels and going into blocking. The fix is to move it to power node C (the unused node in a stock PR) along with a couple resistor and grounding changes.

See this thread for more info and discussion, and specifically my post there: https://el34world.com/Forum/index.php?t ... #msg372357

Also attached gutshot and schematic for reference so you can see what I changed. I had great results doing the mods listed. Hope this helps!

[2pupwonder]

Thanks Adrian, very interesting. I had actually seen your EL34WORLD post a few months ago, but dismissed that as my issue.

However, last evening i rechecked my amp by pulling out v2, and most of the nastiness went away. I still have the greasy distortion PR's are known for, and a little bit of blatty clipping at the highest power levels, but the really ugly stuff is maybe 10% of what it was, even with my LP into input 1.

I previously pulled V2 without much luck, however that was likely before i added the v3b 470k grid stopper.

My amp is relatively quiet, very little hum, some hiss. I have mostly metal film resistors in it, except for the 3.3m on reverb mix is a carbon comp, and the 470k reverb attenuation is a carbon film.

I will look into the reverb mod. Looks like would need an additional parallel filtering stage cap and some resistors.