Pentode phase inverter

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Pentode phase inverter

Australian valve guitar amplifier practice of the 's, taken stage by stage from the input sockets to the speaker socket. A guide for repair and design. The XLR or Cannon-type connector is clearly a better audio connector than a 6. Inputs range from a single socket connected directly to the grid of the first stage, no resistors or blocking cap Marshall PA ; and up to six Vox AC IIall different.

This example has a fairly low input resistance of k but is otherwise direct-connected to the input. This low input resistance would be most unsatisfactory for an acoustic guitar with a direct piezo pickup and would sound very thin. If more than one input socket is provided it is normal that they will have different characteristics. When the input is plugged into the upper socket the input has 1M to ground and two 68k in parallel, 34k, in series.

In the lower input the input resistance is two 68k in series, k, and divides the input in half -6dB. The first would suit a guitar magnetic pickup direct, and the second is effectively a line level and impedance input, so it would suit a stomp box output.

The cathode bypass capacitor on the lead channel is much lower value, 0. The outputs are mixed downstream. Another arrangement is three inputs, one flat for keyboards, one bass cut for lead guitar, and one treble cut for bass Playmaster.

pentode phase inverter

Switch contacts on input sockets are used in many different, cunning and obscure ways to alter input impedance as well as sensitivity TrentStrauss. Ideally the common of the socket should be isolated from the chassis to prevent earth loops. Commonly available isolated sockets are not nearly robust enough for guitar input service.

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XLR's are tougher, but they don't come with switch contacts. A discussion of how to get around this grounding problem. Simple amps have only one amplifier channel even when they have a lot of input sockets. Different signals are mixed early in the chain and amplified together. Normally guitar amps are used by only one instrument at a time and multiple input situations are 'emergencies', so this is not important. More complex amplifiers provide two or more channels.

These were then mixed into the main power amplifier. A more modern idea is to have channel switching in the amp that allows selection of two or more groups of front panel settings.

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There is no reason why channel switching cannot be applied to a valve amp if desired, but it is important to have an 'at-a-glance' indicator to show you which channel is selected. The first amplifying stage in guitar amps are pretty much the same, half a 12AX7 in an almost standard stage. The 1Meg input resistor provides a light loading for the instrument, and while it's still a bit low for accoustic piezo pickups it is adequate.

This was not a great choice since they only have around 2 volts on them in operation. Failures may be rare but they may not be bypassing audio to their best either. There may be some benefit from using a voltage rating closer to the actual operating voltage.

As a rule of thumb electros should operate at more than half their rated voltage so they 'form up' correctly. Another input stage was the low-noise pentode, popular in microphone amp use in PA's and the like.

The EF86 shown here was a specific low-noise front-end amp, but other pentodes such as the 6AU6 have been used. A pentode provides considerably more voltage gain than a single triode stage e. The single-point earth was literal with these stages, all earth returns brought back to a single common point near the valve base, sometimes a metal cylinder in the middle of the valve base.If you have not been trained to work with high voltage then have an amp technician service your amp.

Never touch the amplifier chassis with one hand while probing with the other hand because a lethal shock can run between your arms through your heart. Use just one hand when working on a powered amp. See more tube amplifier safety info here. The following mods are not specific to any particular amp model and apply to many tube guitar amplifiers. Change Tone Cap Value. Baxandall or James Tone Stack. Get overdrive at lower volume and control power tube overdrive.

I love this thing. Switched Cascaded Channels for High Gain. Add a 6" Practice Speaker to your amp head. This easy mod can tune the tone. The 'No Feedback' position makes the amp break up early kind of like a 5E3 Deluxe. The extra feedback of the Heavy position makes the cleans cleaner and tightens up the boundary between clean and overdrive which can make it easier to control breakup with finger technique.

It also makes the amp's overdrive tone cleaner, tighter and more Marshallesque. When changing the feedback source from one output transformer secondary to another you change the feedback resistance by a factor of 1. In blackface and silverface amps I recommend removing the original resistor and connecting the switch to the two now empty resistor eyelets. Heavy is 2. To add adjustable bias to fixed bias amps with no bias pot you simply replace the bias circuit's second, larger resistor connected to ground with a minik linear pot or trim pot and a resistor of about half the value of the original resistor.

The two bias resistors in a typical bias circuit form a voltage divider to reduce the voltage coming out of the bias rectifier diode. One or two electrolytic caps are filter caps to smooth out the pulsing DC from the bias diode. If you find you run out of room and need to get a hotter bias then decrease the value of the second resistor.Log in or Sign up.

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The Gear Page. Aug 4, 1. Messages: I've seen the 6U8A used as a preamp tube with the pentode section as the preamp and the triode as the PI. Here is my question. Should I out the triode first going into the pentode then to the ECL86 gain stage or visa versa? I've found schematics online going pentode first, but would this overdrive the 6U8A's triode cranked up before sending the ECL86's into overdrive?

Seems like since the pentode has more gain, it would be better as a second stage. The reason I want to add more gain stages is I want to add a tone control and I think it will suck to much out of the gain the amp has as it is. Last edited: Aug 4, V2RacingAug 4, Aug 4, 2. I'd recommend pentode first into a cathode follower driving your favorite flavor of tone stack. V2Racing likes this. Aug 4, 3. It has the same pinout as a 6U8A. Tron PestoAug 4, Aug 4, 4. Aug 4, 5. Aug 4, 6.

Yes, there will be some difference in performance overall, but I find it's a matter of how they sound. Hell, if you're looking for "perfectly efficient operation" in guitar tube amplifiers, you'll end up in the looney bin!

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To take it another step further, there are many workable subs with no bias or load changes you can roll: 6GH8, 6KD8, and 6EA8 tubes are other choices with the same pinout in addition to the 6BL8 and 6LN8 tubes. Aug 5, 7. Messages: 7, HotBluePlatesAug 5, Aug 5, 8. I've been thinking triode first, but found it odd all the schematic I found had the pentode first.

It's a simple enough amp and simple enough to add the tube, so I don't really need a schematic. The PT is oversized enough to power another preamp tube easily. There is enough room in the chassis if I just move one terminal strip.There appear to be various names for the phase splitter topology I used; It is variously known, depending on which text you're reading at the time, as; the Concertina phase splitter the Cathodyne phase splitter the Split Load Inverter This type of phase splitter and there are several alternative topologiesis cheep, easy to implement, and capable of good signal voltage balance between the two anti-phase outputs.

Amongst it's disadvantages are the absence of voltage gain, and limitations to signal voltage swing, and relatively high output impedance, with the potential to significantly misbehave should the subsequent output valve s be over driven into grid current.

An intermediary stage, between each of the two phase splitter anti-phase outputs, and the grids of the following output stage valves, is regularly seen in the literature; the Williamson circuit is a classic example.

Such a stage is added to reduce the loading on the phase splitter itself, confer some additional voltage gain, and isolate the phase splitter from possible variations between anode and cathode circuit loads.

I elected not to add such an additional stage, but to try to drive the phase splitter reasonably hard that is, with a high quiescent currentinstead. I have read discussion on the apparent difference in source impedance between anode and cathode outputs. Semantics, definitions of what is actually being considered and meant, and considerable mathematics, all seem to be to the fore in such discussion and, at times, not a little passion and vitriol too!

pentode phase inverter

There does appear to be an apparent paradox. The cathode output, having lower apparent source impedance than that at the anode, may intuitively be expected to have better high frequency response, for a given amount of capacitive loading by the subsequent stage.

The implication is that this would give dis similar phase and frequency performances between the two phase splitter outputs, and hence distortion. However, with the critical proviso that the loads are at all times equal, the output voltages and frequency responses are at all times equal.

The apparent paradox is resolved by considering both the source impedances and the effect within the valve of each load impedance on the voltage of the opposite terminal.

For the special and particular case of both anode and cathode loads being equal, these two effects are self compensating for each other. Ray Moth, that it is again available here. From time to time I see circuits where the anode and cathode resistors have been deliberately made slightly unequal.

pentode phase inverter

I presume this is a supposed design countermeasure to the supposed dissimilar performance characteristics at anode and cathode. If so, then said countermeasure is in fact the problem rather than the cure! This, and the desirability of running as high a quiescent current as continuously tolerable, pretty well fix the design. The cathode resistor is sometimes seen split, in capacitivly coupled circuits, to give a cathode resistor portion to which the grid resistor is returnedof suitable value for self-bias.

In direct-coupled application, as here, neither grid nor cathode self-bias resistor is required.

pentode phase inverter

At near or partial overload, there exists readily apparent asymmetry in the circuit. Significant distortion is likely, predominantly even-order in nature. The clip is approached slowly and progressively, so the overload distortion characteristic is low-order rather than high-order, harmonic content dominated. This is perhaps one of the reasons for favouring other types of phase splitter!

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I suggest that overload characteristic is amongst the factors that give an amplifier its sonic character. I suggest that even during listening levels well within an amplifier's capabilities for all intents and purposes, brief overloads or near-overloads do, and regularly, occur, without is being outright aware of them as singular events.

I hold that such, albeit brief, events contribute to the sound-clues that yield overall "sonic character" or sonic signature". Probably the simplest type of RC phase inverter is made by simply putting half of the plate load in the cathode circuit.Radford's hi-fi amplifiers enjoyed a reputation for quality that was "second to none", especially because of their output transformers.

Radford began building amplifiers inand marketed its first hifi-amplifier in As an expert in transformer design, Arthur Radford was able to reduce distortion by a factor of ten over the best performing amplifier of the day. After many years of work he eventually took out a patent in conjunction with Dr A R Bailey for the first transmission line loudspeaker. This system was the basis for all his larger designs, and is still used by many well-known manufacturers.

Although today, the valve audio amplifiers attract the most attention from enthusiasts, the quality of Radford's designs continued into the transistor age with the "Zero distortion" series of solid state power amplifiers produced in the mid s, in which THD was reduced to less than the circuit noise level. These, and later products such as the Quad [4] and Halcro range, achieved the near perfect measured performance for which designers had been striving since the earliest days of the industry.

Left with no potential for objective improvements in quality, the industry started to concentrate on subjective analysis of designs and products which has, in some cases, led to a revival of interest in valve amplification amongst audio enthusiasts.

Another well known Radford product was the "Labpak" low voltage power supply, used in schools and colleges, both in the UK and overseas. As Arthur approached retirement, the company was wound down as there was no one to take it on. After he retired inWoodside Electronics was formed by John Widgery, who had been apprenticed to Radford Electronics and was responsible for many of the classic designs. Mike Davis joined John as a partner inand they worked together to launch the CD player and update the range of valve equipment as part of the Renaissance series.

Arthur died on 21 Novemberand Woodside continued for a while but finally ended production in Llanbedrog, Pwllheli, North Wales in Radford valve amplifiers are rare today, and still command high prices on the secondhand market.

Much sought after and enthusiastically rebuilt, they are particularly known for classic and restrained physical design and outstanding, listenable sound quality - including a particular ability to drive, very sweetly, the notoriously difficult Quad electrostatic loudspeakers. Small scale production is now carried on by Radford Revival, a UK craft business dedicated to replicating the classic amplifiers of the company's heyday.

From Wikipedia, the free encyclopedia. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Hunt 3 July The Audio Standard. Retrieved 19 February Categories : Audio amplifier manufacturers Audio equipment manufacturers of the United Kingdom Loudspeaker manufacturers.

Hidden categories: Use dmy dates from August Use British English from August Articles needing additional references from May All articles needing additional references. Namespaces Article Talk. Views Read Edit View history. Languages Add links. By using this site, you agree to the Terms of Use and Privacy Policy.Discussion in ' Audio Hardware ' started by wagsFeb 9, Log in or Sign up.

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Steve Hoffman Music Forums. Bad tube Discussion in ' Audio Hardware ' started by wagsFeb 9, Location: Pennsylvania, USA. Can you tell us how stock your ST is? Because the chances are, if most of the parts have never been replaced, your problem lies in the power supply. This is the only part of the amp shared by both channels. In particular, if the quad capacitor is the original, I would wager money that this is your culprit.

It happened to me Sometimes you can tell just by visual inspection. If you write back with more info maybe I could be of further help. Location: North Carolina.

'62 Princeton Phase Inverter Problem

Dougs tubes dougs tubes is a great place to buy tubes. RobsterFeb 9, HGAFeb 9, Location: San Jose, CA. Ryan, a hearty welcome to the Forum. You can expect now to be asked a few questions - we have more than a number of members who own the sort of amps you specialize in. MetrallaFeb 9, Location: Somewhere in the middle. Thanks for the welcome, Geoff!Log in or Sign up. The Gear Page. May 21, 1. Messages: Hi: So I took my brown Fender Princeton to my tech for a general looking over and to see if he could do anything about a kind of harsh fuzzy nastiness I was hearing when the amp distorts.

He identified the problem. The signal is clipping much earlier and much more sharply on one side of the phase inverter than on the other side. Does anyone know what would cause this? He tried a bunch of remedies and tested all the caps for leakage and such but so far hasn't been able to fix the problem. Any ideas of what to check? PlumBobMay 21, May 22, 2. Messages: 30, When you say that this guy's "your tech", what exactly are his qualifications?

Has the amp every worked correctly for you? If so, what did you change if anything? I assume he's tried replacing all tubes? How does he know one side is clipping more? Has he isolated the PI from the power section to see if the problem persists?

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Has he checked all voltages against the schematic? Subbed in new caps around the PI? Checked all resistor values around the PI? Blue StratMay 22, May 22, 3.

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The tech is a local guy who has done some good work for me. This problem with the Princeton has been there since I bought the amp. This is the first I've had someone look at it. As far as I know, my guy has not isolated the PI from the power section, so that's one thing. Here's what he says about what he has tried so far: The phase inverter of the Princeton is distorting the signal on one side of the signal way before the other.

The one side clips soon and relatively abruptly as the amplitude of the signal is increased. The other side squashes gradually with increased amplitude. This is what I did to try and fix the problem: I checked the coupling caps for leakage with a voltmeter, and also temporarily replaced them.

No change. Replaced the 56K ohm plate and cathode resistors with k ohm resistors to see if that would 'center' the waveform for symmetrical clipping.


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