Ah right that explains it.  From the circuit diagram of my amp.  Wishing to feed the 150k plate resistors with 250v rms or approx 350v peak.  The amp was designed to drop about 100v across the plate resistor so the tube was fed at about 150v.  Therefore plate current is about 100/150k or 0.7mA.  This checks out on the valve curves using grid voltage of -1.25v and a cathode resistor of 1k8, gives voltage of 145v and a current of 0.7mA.

Now on to the spice load resistors.  Require 250v at 0.7mA which means they should be 357k.  Selecting nearest standard value of 330k to load each stage, I can change the values of the filter resistors R2 and R5 to get 350v peak on the traces.

Hi Roly,

Also, if you have time, would it be possible for you to do an LTSpice on the PSU, I have modified the PSU to have only two points, for the two stages of the preamp
I have done one but I am not sure of the resistors I have in place.  What I'm actually struggling with is how to load each stage in LTSpice, so that I get an indicative voltage.  I've put a 1K resistor to GND at each filter stage.  Is this correct?

Cheers
Chris

Hi Roly,
Thanks for the input.

Re: PSU

The PSU shown is for the preamp only, due to the fact that it has powered by a 16VAC @ 1.25A wall wart.
I was going to build modular, so the other units will each have their own PSU.  May be a little OTT for each stage to have its own PSU but as you said:

• Avoids the in phase/decoupling issues
• I can take my time to build
• I can spread the cost of the build over a longer period of time
• Makes debugging easier
• Makes refinements/alterations to circuits easier
• Makes repair easier, should anything go wrong

I was also going to simplify the PSU.  Originally when it was first designed, it used back-to-back transformers and it did have filtering points A, B, C and D.  Now that I am using a wall wart for the preamp only, I can dispense with the other filtering points.  I just haven't got around to modifying/simplifying the PSU circuit yet.

Also the fact that we are discussing AVA106, the 450V hextupler off-line for the quad of 6L6GC, negates the use of the PSU shown for the PI/PA.


Re: Input Impedance

I'll probably just go with a 1M resistor rather than the input drive control. As you mentioned previously only really duplicating the effects of the volume control on the guitar and as you also said, still probably not as good as the 5M/FET combination that you have been developing.
However, having said that, with the input control and the unit being built as stand alone, there is the option to be able to use it with other devices such as a cd player.  Originally I was considering duplicating the preamp for stereo channels for hi-fi listening. That's why the heaters of the 12AX7 are running 6.3V and elevated, as I am sure you are aware, common practice in the audiophile world.  However, I haven't decided whether I am going to do this yet.  In all likelihood I won't and therefore will change the input drive as suggested.


Re: Overdrive

I can't for the life of me find the circuit diagram that I used for this part of the circuit.  I know that it came from the EL34World/Hoffman amp site and was labelled as something like a Blues Amp.  I am trying to find the circuit to determine if I've transposed it incorrectly.  Ah, I've just found it Tweed Overdrive Special or Carolina Overdrive and yes you are quite right. I have got it wrong.


Re: Tremelo

Oops my mistake on the labelling.  When I moved the diagram around, I reattached the labels incorrectly.

The circuit was taken from the article:
http://www.preservationsound.com/wp-content/uploads/2012/07/Vibrato.pdf

I know that is says that it can be capacitively coupled to the first audio stage, but I inserted another stage prior to the effects loop and PI because it and the oscillator will be a stand alone unit.
I take your point about the possibility of obtaining sufficient depth and that the LFO/LDR is commonplace these days but I did find the uniqueness of this circuit appealing.  However, uniqueness doesn't necessarily means that it works...hahaha.


Re: OPT
You've mentioned this mod previously and I thought I had included it.  I must have missed it in the subsequent diagram revisions.

Updated the diagram and attached

Cheers,
Chris

Well now we probably are going over the top  :loco

Sweet, thank you very much.

I was wondering if anyone could help me.  I have a question regarding drawing load lines for biasing  of tubes in an amplifier when there are four tubes in parallel, e.g a fender 100 watt amp.
If two tubes have a plate to plate resistance of 10k ohm and we have an OPT with a primary of 10k, then the load line that we need to draw on the tube/plate characteristics curve is 1/4 of this, or 2500 ohms.  If using 250V at plate, load line should pass through the points  250V, 0ma and 0V, 100mA.  Now if we add a second pair of tubes.  If using the same 10K primary OPT, should a 1250 ohm load line now be used, passing through 250V, 0mA  and 0V, 200mA?

@Phil/Phatt, yeah probably am over thinking it, the type of music means, that the original single pentode driver on the spring tank will do nicely...lol

Thanks for your input Phil, very much appreciated.

I'm just trying to get my head around the use of the op amps and the small OT to provide the drive required  :cheesy:

I must admit that I am reticent to use the op amp type circuit.  This is purely my issue.  I have always struggled to get the op-amp type circuits working and have had much better success with discrete components.  This is not to criticise yours or Rod/ESP's work, I just don't have enough experience with them.  For instance many of the reverb recovery circuits I've seen on the net include an LM386 chip.  When I was in amateur radio, oscillation of these chips in audio circuits was the bane of my existence 

Many of the problems that I fear, could probably be overcome by purchasing the ESP pre-etched circuit boards and I may not have the issues that I am concerned about.

Another thing is the type of music that I listen to/play.  I play punk/hardcore (noise the older generation would call it :lmao: ) so I do not require huge amounts of reverb, just some to add depth to the guitar sound.  So I probably only require short tank/two spring/medium delay that is not driven particularly hard.  Having said that, designing/building for maximum amount of reverb possible means that I can turn it off if I want to

I don't have any tanks yet, that was just the one that the Orange circuit used, that's why I picked that one as as example.

While writing this response, I was searching around the net and found an old Gibson reverb schematic (see below), that uses that OT connected between the HT (17.5V compared with the +/-35 that the Tverb uses) and the collector of the reverb tank driver transistor, just as I suggested between the HT and the BFY51 in the Orange circuit.  The tank used is RV4C which is a 4BB2A1C equivalent in the Accutronics range.

4 = long tank
B = 150 input Z
B = 2,250 output Z
2 = medium decay time
A = both input and output grounded
1 = no locking device
C = mounting side on

Unfortunately, I can't find any info on the TF-1000R reverb transformer, but obviously got to match the o/p impedance of the BFY51 to 150 ohms.

These discussions on this thread have opened up a can of worms.  So much to try/experiment/do.  Might be another ten years of reinventing the wheel.   :duh 

Cheers Nodz

@phatt/Phil,
I was reading the reverb information on the Elliot Sound Products (ESP) and from the information that he presents, I assume that the phabb-tverb circuit that you present is for low impedance coils only.
If a standard coil, like the Accutronics 4FB3A1C is used then discrete componentry is required as the op amps can't provide the necessary drive?
If this is the case, is the drive transformer not required either, as per the ESP page and the schematic of the Orange stand alone reverb unit below?
If you really wanted to, would if be possible to include a reverb transformer into the below circuit and rather than take the output off the emitter of the BFY51 transistor, include a transformer between the A+ voltage line and the collector of the BFY51.  I assume that this would need to be something like a 1:1 transformer as impedance of both transistor and tank is high?

Also while tooling around the net, I found an SS reverb circuit but using discrete components rather than ICs.
Design originally came from a Heathkit TA-27, 20W amplifier with tremelo and reverb.

BTW do you have a circuit diagram for the PS?
Is it a 15-0-15V, 20VA troidal transformer centre tapped that you have rectified and then used the +, - connects of the rectifier to get the +15V, -15V as per Image 1 (although this diagram is for 25-0-25 to give +/- 35V)
or have you used each secondary and rectified and then filtered as per Image 2
or have you used a 18-0-18V transformer and rectified and regulated with something like a 7815, 7915 pair as per Image 3?

QuoteThe amateur has several ace-in-the-hole advantages over the big boys, not least because we are personally attached to the results, and you just can't buy that at any price.  We don't have the time or cost pressures, or accountants second guessing technical or sonics matters.  We have the luxury of time to ponder, think and think again, research what others have done, experiment and tinker, let it brew on the back burner, perhaps for years.  There is nothing I could do as a professional designer I can't do in my home workshop.

This is how my design has progressed.  It originally started, with a single tube preamp and a SE PA for about 4W out.  Over a period of time, it has become quite a developed design, moving to PI with prescence control, I/P drive and O/P volume for the preamp, a PP 10W PA and also with the reverb and O/D ideas being added.  So the thing is still fairly amorphous  :cheesy:

QuoteYes, a reverb line doesn't require a lot of drive power, but it does require some, and the 12AX7 is a bit light-on in that respect.  There are still a number of small power pentodes available which would be a more logical choice, however I agree with Phil that solid-state is a serious contender.  The tank itself utterly swamps any subtle "tube tonality" that might be in the reverb sidechain.

As mentioned in the above post, was thinking of going to something like a 6BM8 pentode/triode device.  However, I will seriously look at the SS option.  If I'm building modular, it might very well be easier with respect to the power supply and componenty and provide me with the necessary tonal device I'm after.

QuoteHaving spent years working on this I am fully aware that there is little interest in SS reverb stuff.

Yeah, I totally agree with this, especially in the audiophile area, although a lot of the newer generations are turning away from valve, due to the cost of the valves, PT and OPT.  Much easier to get a 18-0-18V, 300VA toroidal that can deliver the requirements of a 50W chip amp etc.

I have no problems with SS per se, I was predominantly raised in the SS era and all my amateur radio homebrew stuff, including the audio amps etc has been SS.  I have two practice amps and both are SS, so I am not averse to it 

QuoteMainly because a lot of Modern Valve Amps use pathetic SS reverb circuits and I can certainly understand the casual observer thinking He might need a Real Valve reverb and assumes SS is no good for reverb.  But as I've found this is just not so.  Too late the damage is done and SS rev gets the thumbs down.  The Big names just shoe horn in any cheap *Untested* SS circuit as long as it works the average kid will not know the difference.  If Amp makers actually paid for brains instead of advertizing space A SS reverb can work as well as any Glass power device.

Again you raise a valid point.  Many of the modern amps, use poor quality SS reverb circuits.  The stomp boxes and amp modellers can be as bad, if not worse.  I must admit that I like the idea of both SS and valve reverb, as long as they have the spring tanks and are not computer modelled, although some of the top end amps are starting to improve.

QuoteDon't get me wrong I love Valve gear and would love to own a lot of famous gear but I'll never have the funds.

How right you are.  $2500+ for a Fender twin reverb reissue.  I will never own any of this type of gear either.  So bang for the buck when I make my own is important.  As I said the amp itself will be modular and I was thinking of putting it in racks.  The reasoning behind this is I can put all the racks in a case and I can develop bits and pieces of gear as I go along.  I will probably build the 10W 12AQ5 PA to start with but later I might try one of the 50W chip amps for an SS PA.  So an SS reverb is not ruled out either.

QuoteTry looking at some of these as options;
http://sound.westhost.com/project34.htm
Rod Elliot was a huge help while I struggled to come to terms with reverb circuits.  As should be evident from my Reverb circuits I used his mixer setup as it worked straight up, unlike most of the *Claimed to work* stuff found on the net.

I am familiar with the ESP stuff.  I was looking at building the solid state preamp and the 60W? amp at one point.

QuoteI'd err a bit as I don't think an AX7 would have what it takes to drive the tank. (if that is the one you refer?)
An AT7 has ~ x10 the grunt of AX7 as can be evidenced by looking at fender circuits where the AT7 has the plates strapped to drive the tank Tr.

Yeah, I noticed this and was then toying with the idea of going with somethig like the 6BM8 pentode/triode set up.

Quote
Re the loop;
The Tverb has both a true bypass (Front panel) as well as the mute foot switch (back panel).
Depending how you setup the whole chain of events The mute is all you may need.
I only use the true bypass as a back up in case of trouble with a rig setup.

Will check this out.  As I said, open to suggestions and there are no hard and fast criteria with the actual final configuration. 

Cheers

Very nicely done.  However, I was going to build a stand alone tube reverb, using a single tube, similar to these

http://www.el34world.com/Forum/index.php?topic=7957.0

Don't know how they'll go.

Going to use an effects loop bypass unit, so that it's not in circuit all the time like the units shown.

http://www.beavisaudio.com/techpages/PedalHacker/

The one with the 3PDT and indicator light.

Building in a modular fashion over a period of time, so would like everything to match.  19" racks is probably the way to ensure that the cases are always available.  Agree with you about the height, even with using toroidal transformers, still think you will need at least 2U cases.  I want to build a PS, preamp, reverb unit, overdrive unit and power amp in separate cases.  I don't think the reverb tank will fit in the 19" case either, so I might struggle with them.

Edit:  Just had a look at the Accu Bel site for the different types of tank, the outer dimensions of the long tanks are 16.75" so a 19" rack should be alright.