Breadboarding a modified Princeton Reverb amp

[hesamadman]

I just finished installing the power supply and output tube components on my breadboard build I am starting. I had a question about the bias of this amp. First off, when I do breadboard work, I like to do it in sections. I do the power supply first then check voltages (I know they will be high due to no tubes installed and no load) but this at least lets me know if my bias pot is functioning and lets me know if something is going to explode here. From there I install the PI then I will load the tubes and put an input jack on the PI input, hook up a guitar and play around a bit to try and hear if I have any loose connections. I have had a lot of times in the past when a loose connection caused a lot head ache. From here ill do preamp, then add the reverb and vibrato.


So my first question is this. The vibrato intensity pot is in line with bias supply. My bias supply should be fine if I go straight from the 10k bias pot to the pair of 220k resistors at the grids correct? And I will just add the vibrato circuit later. The intensity pot here has always confused me. I never understood how this works. It seems like it would adjust the bias voltage as the intensity pot is adjusted. Voltage drop across the resistance of the pot. Is the reason this doesnt drop voltage because there is no current?


In addition to this, I only recently realized that this is vibrato and NOT tremolo. I have not built anything with vibrato. I understand trem is amplitude based and vibrato is frequency based, but my goal with this amp is to achieve the very nice cleans that the Princeton is known for and I am also going to add in a switchable gain stage for a nice lead tone. I cant imagine the necessity of having a gain heavy lead tone and vibrato at the same time but will adding this stage affect the vibrato or reverb circuit. I am most likely going to try and add the stage at the beginning of the circuit.

[Willabe]

I understand trem is amplitude based and vibrato is frequency based, .....

True.

.... I only recently realized that this is vibrato and NOT tremolo. I have not built anything with vibrato.

No, PR amps have bias vary tremolo on the output tubes.

[hesamadman]

 OK so it is tremolo?   I can't distinguish the difference based upon the schematic I was just basing it off of what it is labeled on the schematic I have and it is labeled  vibrato.  However on the Hoffman design it says tremolo and now that I look at it   

[Willabe]

Yes, it is tremolo (bias vary trem) in a Princeton Reverb. Doug has it labeled correctly.

Leo, for some reason, switched the 2 terms, tremolo/vibrato on his amps and guitars.

Look at the head stock of a Strat and the decal say's tremolo but on his amps he labeled it vibrato.     

[sluckey]

Is the reason this doesnt drop voltage because there is no current?

Correct.

[PRR]

> the head stock of a Strat and the decal say's tremolo but on his amps he labeled it vibrato.

Stretching the strings changes Pitch.

Messing with the tubes changes Volume.

Different effects, though at the rates we use there is really some of both in both techniques.

Before Leo re-invented the language:

Vibrato is a violinist's finger-wiggle (pitch). Singers do the same, slewing held-notes around the exact pitch. Same as guitar string "bending".

Tremolo is shutters on an organ's pipe-cabinet, originally a simple mechano-acoustic volume control but evolved into a few-Hz variation (volume and overtone). Very similar to biasing tubes or an LDR toward cut-off.

[PRR]

BUT-- (I did not know this), in Theater Organs, trem is a air regulator with a wobbled pressure setting. While this does change Volume, the nature of an organ pipe means the Pitch also wobbles with pressure. In a big reverberant room, musicians may note the pitch change more than the volume change. Leo may have understood "trem" this way.

[hesamadman]

I made some more progress today on this breadboard build. I hit a stopping point due to time availability. It may be a few days before I get to mess with it again so I thought I would share with you guys what I bumped into.


I have the power amp completely installed. For some reason I am only getting about 325 volts just after the solid state rectifier. I have 295/295 before. I realized there was a problem the moment I tried to adjust bias. Only had about 295 on the plates of the 6L6 and bias was not working correctly at all. It was averaging in the range of about 150-200mv. I replaced the 470k bias range resistor with a 470 ohm 3 watt since my PT has a 45v bias tap. But this was the only change I made to the hoffman layout obviously other than not having a preamp or trem or reverb installed.

[pompeiisneaks]

> the head stock of a Strat and the decal say's tremolo but on his amps he labeled it vibrato.

Stretching the strings changes Pitch.

Messing with the tubes changes Volume.

But I have a Vox AC30/4 that supposedly  has both trem and vibrato, by a pull swich, and it is created by tubes, or is that not real vibrato and just a different kind of tremolo?

[BetterOffShred]

Yeah I think it's just semantics of the language    Traditionally like PRR said, Vibrato is a technique employed by the player of the stringed instrument to vary the pitch slightly and usually rapidly (also used by singers) using a bending motion.   The term was added to the effect of volume swell/decrease which I personally refer to as "Tremolo".. 

Realistically if you say your amp has "Vibrato" most people understand it's the volume swell thing, not your amp pitch shifting your notes for you.   

As for the AC30.. I found this bit of Info from Ampage

There are two circuits working together -- the oscillator and the modulator portions. The oscillator is very simple so let's tackle this first. It's a straight-forward phase shift oscillator, which works by placeing a series of phase shift networks around an amplifier. The phase changes so that the negative feedback becomes positive and -- viola -- we have an oscillator. The output is sent via the depth preset trim to a polarity or "phase" splitter. The two opposite polarity signals are used for the modulator. 
 
Okay, the modulator. Starting at the input jack, there is a traditional preamp stage (common cathode) with a split load, which drives the input of another polarity splitter. The cathode output is fed to a pair of band-pass filter networks, which are tuned differently. The plate output feeds two different high-pass filters. One each of the high-pass and band-pass filters are summed to provide two different outputs. Each output consists of one high-pass signal combined with one band-pass signal. Each of the final signals has very different frequency response and phase characteristics. 
 
So we have an input signal, which is turned into two signals that have different frequency response and phase characteristics. Those two outputs are re-combined in V9, which also has a split load. Remember our two, opposite polarity oscillator outputs? They are fed to the grids of V9 as well so that when one half of the tube is passing signal, the other isn't. The effect of this is to fade or pan from one of the filtered outputs to the other. The resulting sound is what Vox called vibrato but which is really just phase shifting with some tone shaping. 
 
The Vib/Trem switch shorts the audio going to one of the grids to ground, which removes one of the phase shifted signals entirely. At this point, you have a slightly colored sounding signal turning on and off through V9 or, tremelo. Note that the rate doesn't change when you switch from vibrato to Trem. The switch isn't connected to the oscillator's feedback network. 
 
There's only one more thing to talk about, which is the output of V9. It feeds FOUR series high-pass filters and then the volume control. The purpose of the high-pass filters is to remove DC from the output signal so the circuit wouldn't thump. 

-Brett

[sluckey]

I have the power amp completely installed. For some reason I am only getting about 325 volts just after the solid state rectifier. I have 295/295 before. I realized there was a problem the moment I tried to adjust bias. Only had about 295 on the plates of the 6L6 and bias was not working correctly at all. It was averaging in the range of about 150-200mv.

I suggest you pull those 6L6s and don't put them back in until you fix the bias circuit. Those tubes may already be toasted.

Measure your B+ with the 6L6s pulled. Does it go up considerably?

[hesamadman]

I suggest you pull those 6L6s and don't put them back in until you fix the bias circuit.

I pulled them quick once I realized it. I put them in my high gain build that had el34 and biased it up. Thankfully they work fine. They didnt redplate or anything. I will check voltages again with them out and see.

[silverfox]

Something I've experienced with bias adjustment- If you don't have a good connection with your meter leads it can cause crazy meter readings and it appears as thought the bias is ramping up to 120 ma. After making a good connection the bias reading was within normal range. Yes, there have also been times when I see a crazy meter reading and the plate was dull red too, so I always shut if down until I recheck the meter connections.

silverfox.

[hesamadman]

This is the weirdest things. I have my negative lead of multimeter connected directly to center tap lead, the primary leads each connected to their own 1N4007. Each 1N4007 are connected together at the other end and I am only getting right around 300vdc. I have the bias tap disconnected. No tubes installed. PT completely isolated. But each leg of the PT is 300vac. Both together are 60vac. Why in the world isnt it rectifying correctly?

[SILVERGUN]

You wont see your peak DCV until you've got your first cap connected...
Like this:

[hesamadman]

You wont see your peak DCV until you've got your first cap connected...

Silvergun, youre always a big help. That brought me up to 466vdc. Ok.....now to find my problem down stream. Something silly im sure. It always is.