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Messages - teemuk

Right. And verify that bias doesn't start to climb along with operating temperature. Back down if it does.
Any asymmetrically distorting circuit will produce similar profile. E.g. single-ended common emitter amp.

And no, triode (in a specific circuit) won't do it across a wide range of output levels either: With small signal input the magnitude of distortion is negligible, as usual. With modest overdrive the distortion profile is as depicted. And with harder overdrive the magnitude of distortion increases (this including higher order harmonics), second and third harmonics becoming less prominent overall.

With a push-pull circuit the same triode produces almost no even order harmonic distortion at all, as usual.
You will loose a lot of the 'tubey' characteristics inherent to that amp if you replace the output section with a generic chip amp.

Current limiter is a control for power amp's phase inverter overdrive. Yes, I know it's a solid-state amp. They designed the output secton to follow similar architecture as tube amps (not generic SS) so substitution with chip amp will erase the designed-in tube emulation and PA soft clipping.

Just my opinion.
The Newcomer's Forum / Re: Old 70's Randall Amp
November 12, 2018, 11:05:04 AM
Here's the deal with similarities...

Standel was sold to Randall Instruments, Inc. in 1972 but for a short while the company also continued its venture with the Standel amps as well. Robert Crooks held his position as president, but was also appointed vice president of engineering at Randall. His son was also an executive in both companies.

The cabinets, chassis, knobs, hardware, etc. of early Randall amps... yes, all those parts were pretty much sourced from Standel's inventory. Others, such as transformers, Don Randall had taken with him when he left Fender. They even kept "Standel-ish" cosmetic styling for a while including the "faux" wood panels. Standel largely designed and produced some of the earliest Randall amps (Alpha and Delta series), which were hybrid designs with 8417 tube -based output stages and solid-state preamps.

I would take a wild guess that these designs largely served as a basis for future Randall amp models that followed.

In 1973 Standel was eventually sold to CMI, or Chicago Musical Instruments (also known as Norlin Industries, which actually was the company that owned CMI). Most prominent company under CMI umbrella was Gibson, and Gibson happily adopted Standel's (solid-state) designs as part of their new "G-series" line of solid-state amplifiers and once again they were largely built using the same Standel parts inventory.

Another side venture of that were the "SG Systems" amps, which were mostly hybrids with guess what... 8417 tube -based output stages and solid-state preamps. Yep, another "Standel design". In difference to Gibson G-series amps these largely relied on FET preamp circuitry, which is actually very, very similar to circuitry you'll encounter in an early Randall amp: Same FETs used in very, very similar circuits and overall circuit architecture. Also, once again they largely relied on existing parts inventory, so hardware is similar in Gibson, Standel and (early) Randall amps.

Eventually, Standel operations were seized completely, Gibson stopped building amps for a while under Gibson trademark (I think because they moved their production plant to another state) and then had a brief side venture with "Lab Series" amps. The SG Systems amps on the other hand simply weren't a very long lasting product.

So, Bob Crooks eventually went to work for Barcus-Berry (where he pioneered the concept of "Exciter" products relying on dynamic phase shifting) and Randall simply hired an interesting character called Gary Sunda to take care of amplifier product design. Gary was a former jet propulsion engine designer from Lewis Labs and did all sorts of freelance electronics design (e.g. he had built the power supplies for world's largest "slot car" race ways) and operated an amplifier and speaker repair business. Since mid 1960's he has produced a series of transistorized "Sunda" guitar amplifiers (manufactured by Lewis Labs), which Randall had found quite impressive. After working a shortwhile as a freelance designer for Randall, Sunda was eventually hired on their payroll and became chief engineer. He is largely credited for tweaking the distortion circuitry of Randall amps, and in that he basically continued from where Standel had left. It is possible that Sunda also co-operated in design of those SG systems amps as well.
Judging by the manuals, Fender actually did shortly continue manufacturing Sunn "Stinger" series, at least models 35, 100, 12 and Stinger Bass.

These amps were, however, made in Taiwan, and there's a good chance they were actually products sourced from Fender Japan (which was more or less an individual business entity), thus "OEM" and manufactured by any suitable company on license. Same thing as with the cheaper Squier and Sidekick series, which Fender Japan kept manufacturing despite CBS selling the trademark in USA. These Stinger amps may be - or may not be - similar to Stinger amps produced earlier by Sunn Musical Equipment Co. (by the time a division of Hartzell Corp).
The Stinger series amplifiers are some of the last ones Sunn Musical Equipment Co. manufactured before Fender (FMIC) purchased them. (Which took place in 1985, almost immediately after CBS had sold Fender trademark away and FMIC had been formed by former Fender exacutives). Solid-state design, naturally. Stinger amps were a cheaper following offshoot for more "professional" Sunn amps like SB and SL series, or Stagemaster, Sunn transistor amps that appeared after the more famous "Beta" and "Alpha" series amps. Similar feature to them is using CMOS inverters for overdrive/distortion but these ones lack the "soft limiter" of the power amp stage and probably feature "passive" tonestacks. They are of different design.

I have schematic for Stinger 60 amp. It's obviously designed to be very "tube-like" sounding; all preamp stages are designed non-linear and the power amp stage is made to have low damping factor. The amp is actually very similar to Sunn "Stagemaster", sans some special features like channel switching, FX loop and "soft limiter". This is my hunch, but these amps may have been designed by someone from the famous Moog "Lab Series" team, they bear so striking resemblance to Lab Series L3 amplifier, combining both CMOS inverters AND servo-biased FETs for overdrive. Never seen anything alike in other SS amps, except in Lab Series L3. Just a guess though. Anyway, shortly after these the company was sold to FMIC and that's the end of that story.

AFAIK, Fender did not reissue any of the late Sunn SS amps, or any Sunn design on that matter. They purchased Sunn mainly to acquire a factory, since they didn't have one as CBS kept all of them when they sold the trademark away. So FMIC just needed a manufacturing plant and resources in US ...and purchased Sunn.

Fender did not continue to manufacture any of the previous Sunn designs, did not reissue them, and they did not even "inspire" any of the Fender (FMIC) amps that followed. FMIC created their own, completely new SS preamp design back then... and even today they still keep utilising it in one form or another. Sunn designs - especially solid-state - went down in history right there and then.

Fender may have relabelled some old Sunn inventory, or sold some of the Fender Japan products under Sunn trademark (for a shor while, maybe) but it took several years after the initial purchase before they actually bothered to revive the Sunn trademark. (Sunn was purchased in the mid 1980's whereas Sunn brand was revived in the mid 1990's). When they did that the products were - as said - just Fender products with Sunn logo. Even the famous "Model T" was - to many people's dismay - totally unlike the Model T amplifier that Sunn had manufactured earlier. So it was just reviving of a trademark, not revival of any older Sunn designs. Too bad, they made many great SS amps in the period between early 1970's and early 1980's.

Yes, Fender likely has several Sunn manuals and schematics, which they have acquired when they purchased the company. Hopefully one for this amp too. But, AFAIK, there never was a Fender "Stinger" amp or even anything similar. The Stingers were strictly a Sunn product. (No, 1980's Fender amps with CMOS overdrive aren't similar, that's just a happy coincidence. Using CMOS inverters for overdrive was probably "fad" for everyone since mid 1970's and totally well known "in the art").
Great video.
Yes, Q7 is "rubber diode", and like you guessed you set voltage drop across it with resitive divider R40/R38.

To increase voltage drop you either increase resistance of R40, or decrease resistance of R38. The circuit needs to generate a voltage drop of approximately 4 x diode forward voltage (ca. 2.4V) to compensate for the four base-emitter voltage drops introduced by the push-pull (darlington) emitter follower buffer.

Yes, a trimmer will work. Just make sure the circuit configuration is one that can't reach to a state where bias voltage would be too high. That would lead to catastrophic failure of the amp. Too low bias voltage won't hurt anything, but it just means more crossover distortion in the output signal.

AFAIK, the circuit - as is - should develop a voltage drop of approximately 2V so it's in the right range already.

I sense that your biggest problem is adequate thermal compensation because forward voltage of the transistor's base-emitter junction will vary in interaction with output device temperature, and insufficient compensation can lead to "thermal runaway", where bias current just keeps automatically increasing as the devices heat up. To the point of failure. You practically want an opposite effect: Increasing device temperature should decrease bias voltage.

I assume that such a simple design as this does not employ any "thermal tracking" schemes and is therefore biased fairly "cold" since the bias voltage will remain "fixed" regardless of device temperature. When the output devices heat up quiescent current will increase some but not in a magnitude to cause thermal runaway as the increasing starts from a state of moderately cold bias to begin with. Benefit is increased reliability with the expense of higher overall crossover distortion.

If you can make the bias circuit "thermally track" it allows biasing "warmer" since the bias circuit will self-compensate for effects of increasing temperature and lower bias accordingly. You often see bias diodes or the "rubber diode" transistor mounted to the heatsink (or close vicinity of the output devices) in order to make them track the operating temperature.

In this amp the bias circuit (and the transistor) are located in the circuit board. They will not track output device temperature and bias voltage remains in "fixed" value defined by ambient temperature around the bias circuit (which will be a lot less than the heatsink's temperature and subject to less thermal shifting during operation).

Much comes to application. Is the amplifier going to operate largely at high output volume levels producing ample output power? If yes the crossover distortion will be proportionally lower magnitude and less audible at those high output levels. If the amp is operated largely at low power output, at modest loudness levels, crossover distortion will be proportionally higher magnitude at such signal levels and therefore more audible and obtrusive.

Since audible distortion seems to bother you in normal use of the amp I would try increasing the bias voltage a notch while monitoring that the operation stays reliable and that the bias doesn't run off when the output devices start to heat up. It's probably going to needs just a tiny increase.

Output transistors should operate in a lukewarm temperature. If you burn your fingers to them they are running too hot. Due to practical inefficiencies of heat transfer the case or heatsink temperature will regardless be much, much less than the actual internal die temperature of the transistor, which is the thing one worries about concerning overall reliability.
So rather than worrying about whether the transistors seem to run too "cool" I would concentrate on checking out that the amp produces its rated output power at reasonably low level of distortion, as it should. If it does - and still runs cool - then you can just congratulate yourself for acquiring a very reliable amplifier. Output devices running "hot" is not a preferred state, it's just a nasty side effect one usually has to live with in practice. The cooler they operate the better.
They are for clipping.

But you don't get soft clipping the way you have drawn the circuit.

Look at the feedback loop; as is there's 100% negative feedback resulting into gain of unity. For both DC and AC. The amps are just unity gain "buffers" isolating their input and output circuits.

Now, the way you have drawn those "soft clipping" diodes there results into negative feedback being removed until diode forward voltage is reached. The amp has gain of nearly infinity (for both AC and DC) until Vf is exceeded, and then signal is abruptly clipped.

I think this is not your intention....

If you want that thing to work you need to create proper feedback loops for those stages. Google non-inverting amp for example.

Now another problem: This thing works with sliding DC offsets, so you don't want any DC gain. DC gain will disrupt the operation of the circuit.
Secondly, the gain levels of the entire power amp circuit are tweaked for a circuit where those amps are just unity gain buffers. Add voltage gain there and probably you start to get clipping distortion in unwanted places. Non-inverting amp with diodes in feedback loop also will not "clamp" to specific voltage threshold. And you want that clamping because that's only thing preventing overdrive of the following power amp stage.

My bet is that adding voltage gain there, and increasing clipping threshold - in any form - disrupts the circuit's operation. It's designed to work optimally as is. If you don't know how to tweak it - and you obviously don't - my advise is to do nothing for it because there's a huge chance you just turn things worse - or turn a fully functional amp non-functional. There's very little you will gain in practice from turning the clipping of one half wave a bit softer. As is, it already clips softer than most genuine tube power amps.

Also, trust me: One thing you do not want from that circuit is excessively asymmetric clipping of those "push-pull" pairs. It will not work properly.

Get back to this when you fully understand how that circuit actually operates.
The resistor develops a voltage drop that is dependent on load current. (I=U/R) Since reactive load, like loudspeaker, is not purely resistive but has impedance (IOW load resistance varies with frequency) the voltage drop across the "current sampling" resistor also varies with frequency.

This consequently varies magnitude of additional (negative) voltage feedback taken from that point. This effects closed loop voltage gain of the amp.

Dynamic loudspeaker has some archetypical impedance characteristics: Due to voice coil's inductance the impedance increases towards high frequencies. In addition, at resonant frequency of the speaker the speaker diaghram moves with greatest efficiency, which means the resonant frequency introduces a very high-impedance load in comparison to nominal load impedance. If nominal load impedance is, say, 8 or 4 ohms the impedance at resonant frequency and at upper high frequencies can be close to 100 ohms.

Google speaker impedance. Should be an eye opener.

You can now imagine a resistive voltage divider formed by the load impedance and current sampling resistor. When load impedance is moderately high (e.g. towards high frequencies and at resonant frequency) the resistive divider attenuates the additional feedback signal significantly and consequently voltage gain of the amp increases. When load impedance is low-ish (e.g. nominal impedance) the divider attenuates the additional voltage feedback signal less and closed loop gain decreases.

Thus the amp has higher voltage gain at higher load impedances. This shapes the frequency response: Gain is boosted at resonant frequency and towards higher frequencies.

Resonant frequency is typically near low end roll off of the driver while inductance increases towards upper high frequencies. Therefore result is boost of bass and treble, or in different terms: a mid-range notch.

If you think about an amplifier circuit with high output impedance (archetypal tube power amp) driving a reactive load it creates a similar frequency response due to similar divider formed by output Z and load Z. Simply, if output Z is high more voltage is developed across terminals of higher ohmic loads than across terminals of lower ohmic loads.

On the other hand, if output Z is very low (e.g. generic solid-state power amps) the divider has very little effect on voltage potential created across load terminals. This is great for high-fidelity since amp's frequency response is basically unaffected by load impedance. One acquires flat response despite impedance characteristics of the load. Very nice characteristic if additional "coloration" of the output signal is a no no.

With musical instrument amps (particularly guitar amps), however, we have grown to be accustomed to low and high frequency boost of a high output Z amp. So it's basically just another equalising circuit in the signal path. Amps without it sound lacking in bass and treble because they have flat response instead of boosting highs and lows.

This is a graph from whitepaper published by Fender's design engineers in early 1980's, which presents the difference of frequency response of high output Z amp (tube amp) and low output Z amp (solid-state) driving a reactive speaker load. In high output Z amp the impedance characteristics reflect in overall frequency response, in low Z amp the response remains moderately flat despite changes in load impedance.

Naturally, most SS guitar amp designers have been aware of this since late 1960's and have designed their SS amps to replicate the characteristic where frequency response is dependent on load impedance. For "Hi-Fi" (and largely for bass guitar amplification) such excessive coloration, however, is somewhat a "detrimental" characteristic. You mainly see this in guitar amps only.

So yes, it is true that this kind of circuit mimics similar behavior of tube (power) amps (but only those those that have high output Z) and likewise affects (closed loop) gain of the amp creating an impedance-dependent frequency response.
Long story short: I will not work the way you have drawn it.
It's not just Elite schematic. Elite just omits distortion circuit and adds a third channel, identical to channel 2.

The tech should at least be able to tell whether the circuit diagram matches your amp or not. It is for Taurus series alright, but there could be different versions of amp models within that series.
Polytone amps were designed very modularly: They featured power amp modules, which were subject to design changes since the beginning. For example, you can likely find a handful of "378" power amp designs. Few are discrete; few have an integrated driver chip, which is now obsolete.

Once in a while they made a preamp design packed with features, which then went to the "flagship" model mostly as is. Other models were then derived from that design, largely by stripping off unnecessary features. This is no different from what other manufacturers were doing as well. For example, "Lead" model could include two channels, fuzz or distortion, reverb and perhaps a vibrato function of some sort. For "Bass" model the design was identical except for stripping away one channel, reverb, fuzz/distortion and vibrato as unnecessary features not implemented.

Therefore Polytone schematics were often just titled "Preamp" and notes were included to describe what parts of the portrayed circuitry various preamps of different amp models actually contained.

For example, this preamp schematic applies to Mini Brute models 1 – 4, Brute models 1 – 3, and Mega Brute and Mega Brain models.

All in single schematic. Ignore parts that simply were not fitted.

Note that this was for '90's Brutes. Polytone also made Brute series in the late 1970's simply by stripping certain features away from their former "Lead & Bass" preamp design. With some searching you can find a pre '77 schematic titled "Lead & Bass" and an identical schematic from 1977 titled Lead / Bass / Mini-Brute. The latter describes what sections of the circuitry are used for MB models 1, 2, 3 and 4.

For that reason, with Polytone schematics, make sure you have the period correct! They sure loved to recycle model names.

Here's partial snippet of Taurus series preamp:
(You can find rest of the schematic from Music Electronics Forum*)
This applies to Taurus models, I, II, III and Elite. Taurus I was a single channel amp with most of the portrayed features stripped, Taurus II and III were dual channel amps. Depending on model, "Midrange" and "Distortion" controls were implemented to other controls using dual potentiometers.
"Elite" was a three-channel amp and if you refer to schematic notes (the schematic IS NOT just for Elite model) channel 3 was same as channel 2 and the circuitry involving distortion feature was omitted from "Elite". Also, not all Taurus models featured the rotating cabinet simulator titled "Modulator". 212 versions of these amps were only ones with the portrayed X-over.

AFAIK, all Polytone amps – except Fusion – are built this way. Fusion is an exception since it wasn't a solid-state amp.

I hope this answered your query.

*Link to "Polytone Schematics" -thread here:
Do also check out and for more Polytone schematics.
Tubes and Hybrids / Re: NAD!
June 03, 2017, 04:12:17 AM
QuoteIs that really how they hooked up the speaker back then?
Right off the O/T winding?

Speaker load is the secondary side circuit so how else should they make the connection? Since I see no features such as extension speaker jacks or alike isn't that the shortest and most logical method to connect the speaker?

Theoretically there could be a speaker out jack and a plug in the speaker wires ...but why? Now its solid connection to load and additionally they manmaged to make the cheapie model even cheaper.

IMO, this one isn't even an oddity. You probably haven't seen those vintage amps yet that have their tiny, tiny OTs riveted right to the speaker frames...?