Schematic Finalization

[fossilshark]

Hi all, I finished putting together the entire schematic for my preamp project, I just have a few quick questions before I start ordering parts.

-I am having trouble calculating R1 and R2 drop resistors for the power supply, I have labeled the known voltages. Can I assume each triode is drawing 1ma off B+ for my calculation?

-Does my effects loop/output section look okay? The output is meant to go to a power amplifier like a peavey classic 50/50 unit. I am worried the voltages on the plates of the effects loop triode aren't correct.

-Is 4x 50uf caps really I all I need for good filtering? is there a better way to add in filter caps?

Any advice/criticism is GREATLY appreciated. Thank you all for helping me get this far.

[PRR]

...-I am having trouble calculating R1 and R2 drop resistors for the power supply....

While the currents "can" be calculated (and V1A can't possibly be sucking 1mA, while some others are more), don't strain your brain. Keep lots of 10k 2W resistors. Tack one in for any B+dropper/filter. See where it comes out.

If far too low, tack a second 10K to make 5K.

If too high, tack a second 10k in series to make 20k.

Any sane number of 12A*7 stages can be "figured" this way in a couple minutes.

[sluckey]

You need a FWB rectifier. Your diodes are drawn wrong. If you put a 5K to 10K between the first two parallel filter caps the B+ ripple will be greatly reduced.

[Latole]

You need a center tap at the heater or make an artificial one
Amp will be more quiet.

[fossilshark]

Any sane number of 12A*7 stages can be "figured" this way in a couple minutes.

PRR great advice as always. I will definitely do this.

 

You need a FWB rectifier. Your diodes are drawn wrong. If you put a 5K to 10K between the first two parallel filter caps the B+ ripple will be greatly reduced.

Got it, full bridge rectifier and add another resistor between the first 2 filter caps, I will add this.

You need a center tap at the heater or make an artificial one
Amp will be more quiet.

Good idea I will definitely implement this.

Thank you guys for those tips.

So now that the power supply is worked out, will my output section be OK?

[kagliostro]

About the artificial ground for the filaments

a pot will act way more better than two resistors

the value of the pot may be 220R 500R

the wiper is to be connected at gound and you must put the wiper at the middle, then turn the shaft right or left till you obtain the less humm that is possible

See Humdinger here

http://www.valvewizard.co.uk/heater.html

May be you can also be interested on Elevated Heaters

Franco

[Latole]

Sometimes pot work better.
Where to stop when building a amp ? Good , better, best....
 

[kagliostro]

Many years ago someone said to a pope who covered 90 years

100 of these days holiness

and the pope replied

Why put a limit on divine providence ?

😉

Franco

[Latole]

Many years ago someone said to a pope who covered 90 years

100 of these days holiness

and the pope replied

Why put a limit on divine providence ?



😉

Franco

Why? Because  money cost.

[fossilshark]

ill slap a hum pot in there if space permits.

I am mostly just worried about the output section of this schematic but if nobody has objections then there is nothing left to do then send it

[Latole]

ill slap a hum pot in there if space permits.

I am mostly just worried about the output section of this schematic but if nobody has objections then there is nothing left to do then send it

I did not study the all the schematic ,only the power supply

[pdf64]

How about posting your amended schematic?

[kagliostro]

I am mostly just worried about the output section of this schematic but if nobody has objections then there is nothing left to do then send it

Well .... you didn't say anything about what happen to the output of your preamp

Is the preamp simply the first part of a complete amplifier or are you planning a Stand Alone preamp ?

if you are planning something to be used as a Stand Alone unit in my opinion you must modify the output, instead, if the circuit you are planning is to be followed by a Power Section that is in the same unit, may be that it is OK

---

Look to this schematic (an interesting way to achieve a balanced output)




Consider your schematic similar to the part that is till the right where is write Master Volume (it goes to the PI of the Power Section)

The part below is a possible way to add a Balanced Output able to drive signals on long cables till effect units or other

Franco

[fossilshark]

Yes this is a standalone preamp. The effects loop is meant to drive any effects pedals. My goal is the output triode of the effects loop (doubling as the output of the preamp) should be line level for a 1/4" unbalanced input of a peavey classic 50/50 power amplifier unit.

[fossilshark]

Fixed the power supply. So will V3B output line level?

[kagliostro]

To me to have a good "drive" of the signal on cables you must use a CF (or Source Follower) as output

like in the send of an FXLoop

so I'll use as output V3A or (but I don't think you require to add gain) a CF that follows V3B 

Many years ago I tried to plan a stand alone preamp using as basis the TOS of our friend Tubenit

I never build it, but you can give a look to the output, I stoled it from Merlin's books

Franco

[fossilshark]

kagliostro, I am going to be honest I do not know what you are trying to say to me through that schematic, I might be too drunk though. Please give me the summary.
-in your professional opinion will my current circuit output (V3B) correctly to a standalone power amplifier unit?

[PRR]

> will my current circuit output (V3B) correctly to a standalone power amplifier unit?

No. The impedance is 39k at the plate, and up to 260k at the pot wiper. One FOOT of cable will roll-off at 21kHz. A 3-foot cable will be distinctly dull even on guitar.

And V3A looks like heavy distortion. Find a decent high-level cathode follower and copy it. (Anybody here have suggestions?)

[glass54]

(Here is a thought, not my design PRR)
I have done something very similar regarding U1B and U2B, it was very successful and low distortion.
My HT was around +280V. C7/C8 was 220nF and R12/R13/R14 all 2W Metal Oxide (because I had them  )
Regards
Mirek

[kagliostro]

On Merlin's books you can find explanations about optimized FXLoop, the send part of those circuits Is what you need to use as output in your preamp

Franco

[fossilshark]

And V3A looks like heavy distortion. Find a decent high-level cathode follower and copy it. (Anybody here have suggestions?)

Ok I see what you guys are saying now. V3A and V3B are valvewizards universal effects loop circuit. But I am open to implementing a better design if it means less distortion.

(Here is a thought, not my design PRR)
I have done something very similar regarding U1B and U2B, it was very successful and low distortion.
My HT was around +280V. C7/C8 was 220nF

This looks like what I need although ill have to add another tube to the circuit. Can I power that circuit with 328V instead of your 280V? I am also confused on the purpose of RV1 and RV2. Would I be able to use this circuit as the effects loop send and then input the signal from the effects (or effects send) directly into this circuit modified to output a line level signal?

On Merlin's books you can find explanations about optimized FXLoop, the send part of those circuits Is what you need to use as output in your preamp

I really have to buy his books when I have a few pennies to rub together but right now I am trying to wrap my head around the AC and DC coupled cathode follower section on valvewizard. Theory voodoo does not come easy to me but I am pretty determined to understand this stuff.

So far glass54's schematic suggestion seems to be my best bet but as is the recurring pattern on this project: I need to modify it to suit this specific application.

Any advice on what direction to head in from here is greatly appreciated. I will continue to stare at valvewizard until it makes sense.

[T Wilcox]

Just a thought but maybe get rid of that Merlin FX loop and put a Dumbleator in its place and nothing else after the D-lator.
You will be able to use both instrument and line level FX as needed

[pdf64]

Given the application, the V3b fx loop recovery stage seems redundant.
Just use the fx send as preamp output.

I don’t see what benefit would accrue from a dumblator?
It’s use of a 250k pot on the send means that the send output impedance varies from 0 to 63k, which seems somewhat non ideal. As cable capacitance will cause high frequency roll off.
Hence the fudge of bright caps on the send and output controls https://el34world.com/charts/Schematics/files/Dumble/Loop.pdf
The existing loop design should be more transparent, ie less signal degradation.

[fossilshark]

From what I gather effect pedal signal level will not be adequate for the input of a power amplifier like a peavey classic 50/50. The signal does need to be stepped up (I believe to +4db)

[glass54]

his looks like what I need although ill have to add another tube to the circuit. Can I power that circuit with 328V instead of your 280V? I am also confused on the purpose of RV1 and RV2.

FS
Just to clarify, you can change your PS voltage and you can use just ONE Tube (type).
RV1 and RV2 are your (James Topology) tone control pots for that particular schematic.
I am going to attach my own tested version built in 2020 with PS of nominal +351V. You may need to consider elevated Heater supply.
It was built as an Fx Send section with preamp before TP1. If you want to use it, I suggest picking it up at C11.
You can also adjust R20 up (say max of 100k or so). Voltage gain can be adjusted a little with R15 (Feedback Ratio)
Distortion of this stage is very low (V2A and V2B), probably better then 0.05% so about 10 time lower than a clean preamp signal.
(I can measure this at a later date if you really need D2/D3 quantified)
Regards and enjoy the experimentation,
Mirek

[pdf64]

…
It was built as an Fx Send section with preamp before TP1. …

Why the 100k send pot?
With 1nF cable capacitance it could be rolling off below 6kHz.

[kagliostro]

Better choice will be a 10K send pot

Franco

[fossilshark]

Just to clarify, you can change your PS voltage and you can use just ONE Tube (type).
RV1 and RV2 are your (James Topology) tone control pots for that particular schematic.
I am going to attach my own tested version built in 2020 with PS of nominal +351V. You may need to consider elevated Heater supply.
It was built as an Fx Send section with preamp before TP1. If you want to use it, I suggest picking it up at C11.
You can also adjust R20 up (say max of 100k or so). Voltage gain can be adjusted a little with R15 (Feedback Ratio)
Distortion of this stage is very low (V2A and V2B), probably better then 0.05% so about 10 time lower than a clean preamp signal.
(I can measure this at a later date if you really need D2/D3 quantified)
Regards and enjoy the experimentation,
Mirek

This is delicious, exactly what I need. Just some clarification questions:
-Would this circuit work for an FX return as well? I need a stage after the FX to step them up to line level.
-I can adjust R17 or R18 to make this work with my B+ as long as I still get +234V and +119V?
-TP2 can be adjusted with R15 to get the correct level out?

[fossilshark]

Better choice will be a 10K send pot

I will try this first and see how it goes.

[pdf64]

Better choice will be a 10K send pot

I will try this first and see how it goes.

It may be beneficial to then also scale down the value of R21 to follow suit, eg to 1k.

[fossilshark]

noted I will try that.

[glass54]

Why the 100k send pot?

Good question PDF. I actually used a 50k Log pot. No problems using even lower value pot as it will extend BW and also lower Source Impedance giving a better noise figure, IF you are trying to squeeze out a better S-N ratio 
Main reason for R21 at 10k was avoid a shorted output of "send" and thus causing stress on V2B.
Anyway FS, now you're loaded with ideas, lets see the forthcoming project results
Regards
Mirek

[pdf64]

… I actually used a 50k Log pot.  …

Thanks, sorry to go on, but why 50k?
Given the application (guitar amp circuit stage, late in the signal chain), I don’t perceive resistor noise to be much of an issue.
Rather a key requirement for an fx loop send is a lowish output impedance. As it’s reasonably foreseeable that it’ll need to drive a 6m cable back to the player. Thereby presenting a capacitive load of at least 500pF, more likely around 1nF, perhaps up to 2nF.
A 50k pot with 10k source impedance feeding it will have an output impedance varying from 0  to 15k to 8k, so driving a 1nF cable, the resulting LPF -3dB corner frequency will drop down to nearly 10k6Hz. So -1dB at 1kHz.
I agree that’s probably not a massive issue, far better than a dumblator’s etc 250k send pot, but we certainly do better.
eg with a 10k pot. 

Another benefit of keeping the source impedance to the cables low is that it’ll be more resistant to picking up interference from external EM fields. And fx loop cables will probably be dangling from the rear panel, perhaps close to mains transformers for the amp and fx power supplies.

[kagliostro]

Avoid interference is the reason for balanced output

the counter part is you need a balanced receiving circuit

But serious equipments use balanced lines

Franco

[glass54]

+1 kagliostro, Properly designed Balanced lines, be they SS or Line Transformer, are still KING!!
But here we are with a Muso's guitar amp with FX loop and pedal board 1.3m away from Loop sockets.
SNR about 60 dB ??? Unbalanced cables in a EMC environment (garden  )
Here's the deal pdf. When I get a chance, I'll fire up the APx525 and do a series of measurements. Both at out/in FX loop sockets and then I'll try to find some mediocre quality (basic) and perform same measurement with cable terminated with say 100k MF. Aim for 5m cables?
Any further suggestions pdf? This may be a VERY good exercise and of interest to others so I'll post results.
Currently I'm a little pressed for time.
Regards
Mirek
 

[pdf64]

A typical delay pedal that might be connected in a fx loop might have an input impedance of 1M and output impedance of 1k, eg https://www.boss.info/uk/products/dd-3/specifications/

Measurements are great and it’s fun using such kit. But what’s at doubt? The interaction of resistance and capacitance is well defined.

[fossilshark]

Interesting discussion.

[PRR]

> might have an input impedance of 1M

I'll argue there is no "1Meg" in audio, outside the box. A mere foot (30cm) of cable, 30pFd, is 1Meg at 5.3kHz, trimming the top zing of e-guitar. 20kHz @ -1dB @ 1 foot is 125k; so is 5kHz @ -3dB @ 8 feet (2.4m), a close back-line. And you never have the right cable, working. That 30' long-line in the bottom of the basket wants nearer 30k ohms max source to deliver full guitar zing.

Also a lot of effects you would use in a loop (instead of a guitar cord chain) come from higher-level traditions and may be 10k or 22k input.

[fossilshark]

Alright here we go again. I have decided to axe the effects loop and just aim for 1Vrms output making it compatible with effects pedals (I'll just build him a switchbox to use as an "effects loop"). I have discovered the Soldano x88r (I live under a rock).

https://el34world.com/Forum/index.php?topic=24959.0 <--- this thread of someone building a clone gave me the voltages. I will attach my v30.56 schematic and the Soldano schematic I yoinked the circuit from. R1, R2, and R3 I will have to just experiment with to get the voltages right.

Please tell me if I get the voltages right I can make this thing spit out a 1Vrms output. At this point if the distortion is low enough to still get clean tones out of it I am not pressed about super tone transparency and it will just have to be my spin on the Chupacabra sound.

Any advice greatly appreciated. Thank you all for your time.